The suitability of active personal dosimeters as the
legal dosimeter for PET radioisotope workers
Steven J Crossley
Supervisors: Dr. Roger Price, Dr. Mike House
Masters Thesis submitted as part of the M.Sc. by Thesis and Coursework
in the School of Physics, University of Western Australia
Date of submission: 28th of September 2016
Abstract
Staff working with PET radiopharmaceuticals wear active personal dosimeters and
a passive dosimeter which provides the legal dose record for regulatory purposes.
Given the capabilities of current active dosimeters with a dose logging capability it
may be asked whether the active dosimeters could be used as the legal dosimeter,
removing the need for a passive dosimeter.
A series of controlled experiments were performed exposing active dosimeters
and two types of approved passive dosimeters to a range of doses from vials con-
taining 18FDG. Reported doses from passive and active monitoring of staff were
compared over 24 months. A questionnaire was used to gauge worker preferences
and acceptance of different personal dosimeters.
It was found that the active dosimeters agree well with the TLD results over
the range of doses tested in the controlled experiments. Agreement with the OSL
dosimeters was not as good. Active dosimeters gave more repeatable results than
either of the passive dosimeters.
There was poor agreement between the passive and active dosimeters in the
worker results for both radiopharmaceutical production workers and nurses and
technologists working with PET patients. Large numbers of the passive dosimeters
reported “below the detection limit” when the active dosimeters reported doses above
the supplier stated detection limits.
Workers were positive in their response to using active dosimeters, and felt that
they were useful in aiding their radiation protection.
Controlled experiments have demonstrated that active dosimeters are capable of
accurately and reliably reporting doses from 18FDG. Comparisons of worker doses
were far less conclusive and demonstrated the difficulty of obtaining accurate dose
data from personal dosimeters of any kind. The main hurdle to the use of active
dosimeters to provide the legal record of worker exposure seems to be regulatory
rather than technical.
Acknowledgements
I would like to acknowledge the assistance I have received form my co-workers in the
Medical Technology & Physics Department (MTP) at Sir Charles Gairdner Hospital.
The RAPID group within MTP, in particular Peter Gibbons and Chris Jones, for their
dispensing of doses of radiopharmaceutical and helping with the ordering, setting up
and running of the active dosimetry system. The Medical Physics group in MTP for
help with ordering and reporting advice for passive dosimeters and assistance with
literature searches and for there feedback during thesis writing. I’d like to thank
Phil Parr and Barry Turk for their mechanical skills in making my experimental
rig. Janette Atkinson and Dr Roger Price have my gratitude for allowing me the
time and resources to carry out my research in the department. The nurses and
technologists in Nuclear Medicine also deserve thanks for their willingness to assist
me in trialing the active dosimeters in their department.
My project coordinator Dr Mike House has been a great help in pulling this
thesis together and in helping me complete the rest of the Masters course. I would
like to thank him for his patience.
Last, but by no means least, my wife Kelly has helped greatly with patience, ad-
vice, assistance and motivation throughout my Masters and indeed our life together.
Contents
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Types of personal dosimeter . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Passive Dosimeters . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1.1 Film Badges . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1.2 Thermo-luminescent and Optically Stimulated Dosime-
ters (TLD & OSL) . . . . . . . . . . . . . . . . . . . 4
1.2.2 Active Personal Dosimeters . . . . . . . . . . . . . . . . . . . 5
1.3 The Accuracy of Personal Radiation Dosimeters . . . . . . . . . . . . 10
1.4 Active Dosimeters for Legal Assessment of Occupational Dose . . . . 11
1.5 PET Radiopharmaceutical Production . . . . . . . . . . . . . . . . . 12
1.6 PET Radiopharmaceutical Dispensing and Use . . . . . . . . . . . . . 14
1.7 PET Centre workers . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.8 Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Experimental Methods & Materials 17
2.1 Radiation Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Passive Dosimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1 TLD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.2 OSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Active Dosimetry System . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.1 DMC 2000 and DMC 3000 . . . . . . . . . . . . . . . . . . . . 20
2.3.2 Logging Station & Database . . . . . . . . . . . . . . . . . . . 21
2.3.3 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 Controlled performance comparison of passive and active dosimeters . 24
2.4.1 Radiation Safety . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4.2 Physical layout of experiment . . . . . . . . . . . . . . . . . . 25
2.4.3 Conducting an Exposure . . . . . . . . . . . . . . . . . . . . . 28
i
CONTENTS ii
2.4.4 Exposures Performed . . . . . . . . . . . . . . . . . . . . . . . 29
2.4.5 Obtaining Results . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.4.6 Normalising results from separate exposures . . . . . . . . . . 31
2.4.7 Displaying Results . . . . . . . . . . . . . . . . . . . . . . . . 31
2.4.8 Statistical Assessment of Difference of Means . . . . . . . . . . 33
2.5 Comparison of staff doses recorded by passive and active dosimeters . 34
2.5.1 Gathering RAPID Staff doses . . . . . . . . . . . . . . . . . . 34
2.5.2 Gathering PET Centre Staff doses . . . . . . . . . . . . . . . . 34
2.5.3 Comparison of doses . . . . . . . . . . . . . . . . . . . . . . . 35
2.6 User Experience Survey . . . . . . . . . . . . . . . . . . . . . . . . . 36
3 Results 37
3.1 Results below the detection limit . . . . . . . . . . . . . . . . . . . . 37
3.2 Controlled performance comparison of passive and active dosimeters . 39
3.2.1 Comparison of doses around the experimental rig . . . . . . . 39
3.2.2 Comparison of results from the same dosimeter type . . . . . . 39
3.2.3 Comparison of dosimeter results with theoretical dose . . . . . 41
3.2.4 Effects of angling the dosimeters . . . . . . . . . . . . . . . . . 43
3.2.5 Comparison of passive dosimeters . . . . . . . . . . . . . . . . 47
3.2.6 Comparison of active dosimeters with OSL dosimeters . . . . . 50
3.2.7 Comparison of active dosimeters with Thermoluminescent dosime-
ters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2.8 Summary of inter and intra-dosimeter type comparisons in
controlled experiments . . . . . . . . . . . . . . . . . . . . . . 52
3.2.9 Statistical significance of agreement of means . . . . . . . . . 54
3.3 Comparison of staff doses recorded by passive and active dosimeters . 55
3.3.1 RAPID Staff doses . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3.1.1 2012 . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3.1.2 2013 . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.3.1.3 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.3.1.4 The effect of reported wear position on correlation . 60
3.3.2 PET Centre Staff doses . . . . . . . . . . . . . . . . . . . . . . 61
3.3.2.1 2013 . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.3.2.2 2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.4 User Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.4.1 Profession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.4.2 Time using Passive and Active Dosimeters . . . . . . . . . . . 66
iii
3.4.3 Ease of use of Dosimeters . . . . . . . . . . . . . . . . . . . . 66
3.4.4 Comfort wearing Dosimeters . . . . . . . . . . . . . . . . . . . 67
3.4.5 Wear Position of Dosimeters . . . . . . . . . . . . . . . . . . . 67
3.4.6 Frequency of checking results . . . . . . . . . . . . . . . . . . 67
3.4.7 Level of trust in dosimeter results . . . . . . . . . . . . . . . . 68
3.4.8 Rate of not wearing a dosimeter . . . . . . . . . . . . . . . . . 68
3.4.9 Usefulness of results and feedback . . . . . . . . . . . . . . . . 69
3.4.10 Prefer to wear Active, Passive or Both . . . . . . . . . . . . . 69
3.4.11 Additional Comments . . . . . . . . . . . . . . . . . . . . . . 69
4 Discussion 71
4.1 Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.2 Equivalence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.3 Repeatability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.4 Limits of Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.5 User compliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.6 User Acceptance of Active Dosimeters . . . . . . . . . . . . . . . . . . 75
4.7 Approval of Personal Radiation Dosimetry Services . . . . . . . . . . 76
4.8 Standards for Personal Radiation Monitors . . . . . . . . . . . . . . . 77
4.9 Calibration of APDs . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.10 Record Keeping and Data Analysis . . . . . . . . . . . . . . . . . . . 78
4.11 Incident investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.12 Economic Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.12.1 Costs of Passive Dosimetry . . . . . . . . . . . . . . . . . . . . 80
4.12.2 Costs of an Active Dosimetry System . . . . . . . . . . . . . . 81
4.12.3 Lifetime of MGP Active dosimeters . . . . . . . . . . . . . . . 81
4.12.4 Comparison of costs per year . . . . . . . . . . . . . . . . . . 82
4.13 Legislative issues in Western Australia . . . . . . . . . . . . . . . . . 85
5 Conclusion and Future Work 87
5.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Bibliography 89
A User Experience Survey 94
B Example Dose Reports 97
List of Figures
1.1 Electron trapping in TLD/OSL . . . . . . . . . . . . . . . . . . . . . 4
1.2 Electron relaxation during TLD/OSL readout . . . . . . . . . . . . . 5
1.3 Doped silicon semiconductor structures illustrating free electrons (n-
type) and electron holes (p-type). . . . . . . . . . . . . . . . . . . . . 6
1.4 Diode with no applied voltage . . . . . . . . . . . . . . . . . . . . . . 7
1.5 A reverse bias diode acting as a radiation detector . . . . . . . . . . . 8
1.6 Example diagram of an FDG synthesis system (IBA Synthera) . . . . 13
2.1 Radiation Detection Company TLD . . . . . . . . . . . . . . . . . . . 19
2.2 Landauer OSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Active dosimeters DMC2000S, DMC2000X, DMC2000XB and DMC3000 20
2.4 Logging Station with dosimeter in cradle . . . . . . . . . . . . . . . . 22
2.5 Logging Station showing dose results at log out . . . . . . . . . . . . 23
2.6 Passive dosimeters arranged on 1m radius rail . . . . . . . . . . . . . 25
2.7 Personal dosimeters on holders. . . . . . . . . . . . . . . . . . . . . . 26
2.8 Dosimeter holders on the rail at 0, 30 and 60 degrees . . . . . . . . . 27
2.9 Example plan layout of experimental setup . . . . . . . . . . . . . . . 28
3.1 Comparison plots for results from the same dosimeter type . . . . . . 40
3.2 Comparison of Active dosimeter results with theoretical dose . . . . . 42
3.3 Comparison of OSL dosimeter results with theoretical dose . . . . . . 42
3.4 Comparison of TLD results with theoretical dose . . . . . . . . . . . 43
3.5 Plots showing the effect of angulation on MGP dosimeters . . . . . . 44
3.6 Plots showing the effect of angulation on OSL dosimeters . . . . . . . 45
3.7 Plots showing the effect of angulation on TLDs . . . . . . . . . . . . 45
3.8 Plot of Normalised Mean Results against angle . . . . . . . . . . . . . 46
3.9 Initial comparison of passive dosimeter results . . . . . . . . . . . . . 47
3.10 Comparison of OSL and TLD results after repeat exposures . . . . . 48
3.11 Comparison of OSL and MGP results . . . . . . . . . . . . . . . . . . 50
iv
LIST OF FIGURES v
3.12 Comparison of TLD and MGP results . . . . . . . . . . . . . . . . . . 51
3.13 Mean dose per Dosimeter Type vs Mean dose per exposure . . . . . . 53
3.14 Comparison of RAPID staff dose results for 2012 . . . . . . . . . . . 56
3.15 Comparison of RAPID staff dose results for 2013 . . . . . . . . . . . 58
3.16 Comparison of RAPID staff dose results for 2014 . . . . . . . . . . . 59
3.17 RAPID staff dose results for 2014 for staff wearing the passive and
active dosimeters in the same position on the body. . . . . . . . . . . 61
3.18 Comparison of PET Centre staff dose results for 2013 . . . . . . . . . 62
3.19 Comparison of PET Centre staff dose results for 2014 . . . . . . . . . 64
4.1 Lifetime of MGP dosimeters . . . . . . . . . . . . . . . . . . . . . . . 82
List of Tables
2.1 Active Dosimeter Models Used . . . . . . . . . . . . . . . . . . . . . . 21
2.2 Alarm Settings on Active Dosimeters . . . . . . . . . . . . . . . . . . 22
2.3 Exposures performed in initial controlled experiments . . . . . . . . . 30
2.4 Exposures performed in repeated controlled experiments . . . . . . . 30
2.5 RAPID Staff Numbers 2012-2014 . . . . . . . . . . . . . . . . . . . . 34
2.6 Numbers of staff wearing active dosimeters when working in PET only
(Nov 2012 to December 2013) . . . . . . . . . . . . . . . . . . . . . . 35
2.7 Numbers of staff wearing active dosimeters while working with PET
and Nuclear Medicine patients (January to December 2014) . . . . . 35
3.1 Reported Minimum Detection Limits for Passive dosimeters as stated
by suppliers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2 Number of excluded dosimeter results in the controlled experiments . 38
3.3 Position dependence of results . . . . . . . . . . . . . . . . . . . . . 39
3.4 Summary of changes in dose readings when angling dosimeters . . . . 46
3.5 Summary of linear fits to comparisons of dosimeter results . . . . . . 52
3.6 Mean difference and 1.96σv values for dosimeter comparisons . . . . . 52
3.7 T-test Results (for p=0.05) for agreement of different dosimeter types 54
3.8 Bland-Altman Results for 2012 RAPID Doses . . . . . . . . . . . . . 57
3.9 Bland-Altman Results for 2013 RAPID Doses . . . . . . . . . . . . . 57
3.10 Bland-Altman Results for 2014 RAPID Doses . . . . . . . . . . . . . 60
3.11 Bland-Altman Results for 2013 PET Centre Doses . . . . . . . . . . . 63
3.12 Bland-Altman Results for 2014 PET Centre Doses . . . . . . . . . . . 64
3.13 Professions of those surveyed . . . . . . . . . . . . . . . . . . . . . . . 66
3.14 Experience using Active and Passive Dosimeters . . . . . . . . . . . . 66
3.15 Ease of Use of Dosimeters . . . . . . . . . . . . . . . . . . . . . . . . 66
3.16 How comfortable are dosimeters . . . . . . . . . . . . . . . . . . . . . 67
3.17 Wear Position of Dosimeters . . . . . . . . . . . . . . . . . . . . . . . 67
vi
LIST OF TABLES vii
3.18 Frequency of checking dosimeter results . . . . . . . . . . . . . . . . . 67
3.19 Level of trust in dosimeter results . . . . . . . . . . . . . . . . . . . . 68
3.20 The rate at which workers forget to wear dosimeters . . . . . . . . . . 68
3.21 Usefulness of results and feedback . . . . . . . . . . . . . . . . . . . . 69
3.22 Prefer to wear active, passive or both . . . . . . . . . . . . . . . . . . 69
4.1 UK HSE Pass/Fail criteria for dosimetry services for monitoring whole
body gamma exposure (Health and Safety Executive, 2010) . . . . . . 77
4.2 Economic Comparison of Active and Passive Dosimetry . . . . . . . . 83
Chapter 1
Introduction
1.1 Background
Exposure to ionising radiation is potentially harmful both in terms of large acute
doses, causing tissue effects, and small but chronic exposure increasing the risk of
stochastic effects, in particular cancer (International Commission on Radiological
Protection, 2007). While the statistics of cancer induction make it impossible to
prove that doses of a few milli-sieverts increase the risk of cancer in humans, data
at higher doses indicate a linear relationship between radiation exposure and the
probability of cancer induction. Radiation safety standards and legislation assume
that this relationship is linear for low exposures all the way down to zero; this is
known as the linear no threshold hypothesis (LNT) (International Commission on
Radiological Protection, 2007). In order to assess the risk from an exposure, or
a series of exposures, it is essential to know the dose to which the individual was
exposed (International Commission on Radiological Protection, 2007), even when
the exposure level is low.
International recommendations have been made to limit the dose to which ra-
diation workers are exposed (International Commission on Radiological Protection,
2007). Based on these recommendations governments, in Australia and around the
world, have put in place laws (Western Australia, 1984; The Health and Safety Ex-
ecutive, 1999; South Australia, 2000) to limit the risks to occupationally exposed
workers from radiation. Dose limits are set to keep the risk from ionising radiation,
calculated using LNT, similar to workplace risks of other kinds, accepted by workers
in other occupations. In order to monitor compliance with dose limits, many juris-
dictions have also mandated the use of personal radiation monitors (The Health and
Safety Executive, 1999; Western Australia, 1984; Bolognese-Milsztajn et al., 2004).
1
1 Introduction 2
Personal radiation monitors are devices worn by individual workers which are
used to provide a permanent record of their radiation exposure. The principal is
that the monitor is exposed to the same radiation fields as the individual, and is
capable of recording the exposure. The dose that the monitor has been exposed to
is then read from the dosimeter to provide a record of the exposure of the individual
(National Council on Radiation Protection and Measurements, 1995).
The materials used to record radiation exposure in personal dosimeters have
evolved over time, but the general working practice has remained the same, with
personal dosimetry being provided as a service by approved suppliers. It has been
argued that developments in active dosimeter technology could change this model.
Employers could provide their own dosimetry service while improving radiation safety
through feedback on dose rates from active dosimeters (Luszik-Bhadra et al., 2007).
1.2 Types of personal dosimeter
1.2.1 Passive Dosimeters
Different types of passive dosimeter record dose in different ways, but all are worn for
a fixed period of time, usual one or three months, by a specific individual and then
returned to the supplier for reading (American Association of Physicists in Medicine
[AAPM], 1995). The dosimeters are recording their radiation exposure from the
time of their manufacture to the time of their reading. The supplier then provides
a report of the cumulative dose for each individual over the period the dosimeter
was worn. The effect of background radiation is mitigated by the use of “control”
dosimeters which come from the same batch as the dosimeters which will be worn.
The control dosimeters travel to and from the workplace with the dosimeters which
will be worn by staff, but are kept away from occupational exposure. The reading
of the control dosimeter is subtracted from that of the worn dosimeters to give
the occupational dose reading. Passive dosimeters provide a retrospective record of
received dose which is reported some time after the exposure occurs. The dosimeter
is sent to be read at the end of the wear period, and there is a delay between the
end of the wear period and the reporting of the dose. This delay can extend to three
or four months (Lummis, 2013). Passive dosimeters have a decades long history of
use in radiation protection, and their performance across a wide range of radiation
energies and types is well understood and documented (Luszik-Bhadra et al., 2007).
There comes a point where a measurement is too small to reliably distinguish it
from background radiation. Passive dosimeters start recording background radiation
1 Introduction 3
from the moment they are manufactured or are reset through heating or exposure to
a strong light source. When a worker is only exposed to small amounts of radiation in
their occupation, this small amount of radiation can be swamped by the background
signal acquired over the months between manufacture and reading of the dosimeter.
Even with background subtraction there is a limit to how small an exposure can
be reliably detected. Because of these issues, passive dosimeters have a minimum
detectable dose below which no reliable dose information can be obtained, and thus
readings below this level are not reported. The value of the minimum detectable
dose varies from provider to provider, but is largely governed by the material used
to record the dose.
1.2.1.1 Film Badges
Film badges are the oldest type of passive personal dosimeter still in use today, but
are being phased out in some jurisdictions, including France and Germany (Luszik-
Bhadra et al., 2007). A film badge contains a small sheet of radiation sensitive film
protected from light by an opaque packet. The film is housed in a plastic holder
that can be attached to clothing. Radiation incident on the film causes chemical
changes which make the film darker when developed, increasing its optical density.
The film is “read” with a densitometer as the optical density of the developed film
is proportional to the radiation dose it has been exposed to. Due to its composition
and density, film does not absorb radiation in the same way that human tissue does.
Exposed to the same radiation, film will absorb a different fraction of the energy
from the radiation than tissue would. Film is not “tissue equivalent”. As the film is
not tissue equivalent the optical density of the film is not directly related to tissue
dose. The use of a range of filter materials placed in the holder between the radiation
source and the film give a range of optical densities on the film. The set of optical
densities can be used in the calculation of tissue dose. A filter is a known thickness
of a pure material with known radiation absorption qualities. If two areas of a film
are exposed to the same radiation source with different, known filters, the difference
in the energy deposited in the two regions gives information on the energy of the
radiation. This spectral information can be combined with the dose to the film to
deduce the dose to tissue. Typically film badges have a minimum detectable dose of
100μSv (Bushberg, 2012). They are lightweight and inexpensive, but easily damaged
by exposure to light, heat or moisture. As the film is replaced in the holder each
month or quarter it is possible to load the film into the holder the wrong way around.
Rotating the film changes the positions of the filters relative to the film, leading to
1 Introduction 4
inaccurate results (Bushberg, 2012).
1.2.1.2 Thermo-luminescent and Optically Stimulated Dosimeters (TLD
& OSL)
TLDs and OSLs are both radiation exposure monitoring devices which make use
of a scintillant material to record exposure over a period of time. Scintillants are
materials which give off visible light when irradiated by ionising radiation. For most
scintillant materials the emission of light is immediate (prompt fluorescence). In
TLDs and OSLs small amounts of specific impurities (dopants) are used to create
electron traps. When electrons in the material are excited to higher energy levels by
ionising radiation they transition to the electron trap rather than returning to the
valence energy level as shown in figure 1.1. Light is only emitted when a stimulus
enables the trapped electron to return to the valence band, emitting a photon of a
particular frequency (see figure 1.2). In the case of TLDs the stimulus is heat, with
an OSL the stimulus is laser light of a particular frequency scanning the surface
of the dosimeter (Bushberg, 2012). The amount of light given off during reading
is proportional to the amount of radiation absorbed. As the materials used in
the dosimeters have a similar effective atomic number to tissue the light output is
broadly proportional to the dose to tissue (Bushberg, 2012). The use of filters of
different materials allows for more accurate determination of equivalent dose, based
on the dose to dosimeter material behind each filter. The use of filters is particularly
important for measuring dose from low energy photons. In modern TLDs and in
OSLs the filters are fixed inside the dosimeters (Obryk et al., 2011).
Figure 1.1: Electron trapping in TLD/OSL
1 Introduction 5
Figure 1.2: Electron relaxation during TLD/OSL readout
From the users’ perspective the badges are handled in the same way as film
badges, they are worn for a given period, and then returned to the supplier. For a
TLD the badge is heated to a particular temperature in controlled conditions, and
the light emitted is detected by a photo-multiplier (PM) tube. The electrical signal
from the PM tube is proportional to the light emitted which in turn is proportional
to the radiation dose delivered to the scintillant (Bushberg, 2012). When reading an
OSL dosimeter the surface of the scintillant is scanned by a laser of one frequency
which causes the de-excitation of electrons, and the emission of light of a different
frequency from the illuminated region (Bushberg, 2012). The scanning of OSL
dosimeters allows for readout of the distribution of dose across the dosimeter which
can give information relating to the nature of the exposure, for example whether it
was a single acute exposure or a number of smaller exposures (Akselrod et al., 2000).
OSL badges can also be scanned more than once if there is a query (McKeever and
Moscovitch, 2003).
Film badges themselves form a permanent record, and can be reviewed if required,
but once TLD and OSL badges are read the badges are stimulated to return all
electrons to the ground state, and the scintillant material is re-used (Bushberg, 2012).
The ability to reuse scintillant materials keeps costs down.
1.2.2 Active Personal Dosimeters
Active personal dosimeters (APDs) contain at least one semiconductor based ra-
diation detector, with electronics to calculate and display equivalent dose. When
semiconducting material absorbs ionising radiation, electrons are promoted to the
conduction band from the valence band, creating electron-hole pairs in a manner
similar to that in TLDs.
1 Introduction 6
Without cooling, applying a voltage across a pure semiconductor to collect the
charge carriers induces a greater number of electron-hole pairs than low doses of
radiation. This renders pure semiconductors inefficient radiation detectors at room
temperature. To overcome this problem a semiconductor diode is used with a reverse
bias. A diode consists of an n-type semiconductor which contains mobile electrons
joined to a p-type semiconductor containing electron holes. The effect of dopants on
the crystal structure of a semiconductor is shown in figure 1.3.
The free electrons and holes are present due to the presence of dopants with
fewer or more valence electrons than the semiconductor material itself. If an element
containing one more valence electron than the semiconductor is present, the spare
valence electron can act as a free electron in the structure of the semiconductor. The
presence of an element containing one fewer valance electrons than the semiconductor
will create a hole into which electrons can move, the movement of an electron to fill
the hole creates another hole. The hole thus acts as a mobile charge carrier in the
semiconductor.
Figure 1.3: Doped silicon semiconductor structures illustrating free electrons (n-type)and electron holes (p-type).
The potassium atom (P) contains an unpaired valence electron, and the boron atom (B)requires an extra electron to form bonds to all the surrounding silicon (Si) atoms.
1 Introduction 7
Figure 1.4: Diode with no applied voltage
With no voltage applied to the diode the mobile charge carriers are distributed
through the diode (figure 1.4). When a reverse bias is applied the mobile charge
carriers move to the edge of the diode, leaving a region at the junction of the two
semiconductors free of charge carriers; the depleted region (figure 1.5a).
When exposed to ionising radiation, electron-hole pairs form in the depleted
region (figure 1.5b). Moved by the applied voltage, the charge carriers generate a
small current which can be amplified and measured (figure 1.5c).
1 Introduction 8
(a) Diode with reverse bias creating the depleted region
(b) An incident gamma photon creating a charge pair
(c) Movement of the charge pair generating a small current.
Figure 1.5: A reverse bias diode acting as a radiation detector
A diode detector acts as an ion chamber. The current generated is proportional
to the energy deposited by the radiation, which is proportional to the dose to tissue
(Bushberg, 2012). As the signal is electrical it can be used by computing circuits as
the input to calculations, the results of these calculations of radiation dose can be
1 Introduction 9
recorded, and displayed to the user in real time.
As active personal dosimeters can give instant feedback on the dose and/or dose
rate, they have primarily been used for operational radiation protection monitoring
(Ginjaume et al., 2007). In particular they are used where there is the potential
for high dose rates, necessitating immediate feedback to minimise exposure. Their
use is mandated for some occupations in Western Australia (Radiological Council of
WA, 2010), including radiochemists working with large activities of PET isotopes.
There have been a number of papers published comparing the performance of
many of the available APDs. The IAEA established a methodology for comparing
the performance of APDs over a range of x-ray, gamma and beta energies, and
compared 13 different dosimeters (International Atomic Energy Association, 2007).
Dosimeters from Artomex, Canberra, Graetz Strahlungsmesstechnik, Polimaster,
SAIC, Synodys Group (MGP and Rodos), Thermo Electron and Unfors were tested.
All but one of the tested monitors reported Hp10 (the personal dose equivalent to
tissue at a depth of 10mm, usually referred to as deep dose), but 3 (Atomex AT3509B,
MGP DMC2000XB and the Thermo Electron EPD Mk2.3) also report Hp0.07 (the
personal dose equivalent to tissue at a depth of 0.7mm; the surface or skin dose).
The Unfors NED is an extremity monitor used for monitoring the dose to fingers or
eyes, and only reports Hp0.07. The monitors were tested using a range of different
radiation sources facing the radiation source, and at 30 and 60 degrees from the
source direction to assess that response is acceptably independent of direction.
The IAEA report concluded that the performance of the active dosimeters was
generally comparable to that of passive dosimeters when measuring gamma radiation,
but only a few accurately reported beta and low energy x-ray radiation doses. Many
of the dosimeters were incapable of measuring pulsed x-ray doses accurately; this
failure has also been reported in other publications (Ambrosi et al., 2010; Ankerhold
et al., 2009; Bordy et al., 2008; Clairand et al., 2008). Pulsed x-rays are used exten-
sively in fluoroscopic procedures in hospitals, and so the selection of an appropriate
model of dosimeter is critical in this setting.
It is clear from the literature that the choice of active dosimeter should be carefully
considered to ensure that it is suitable for dosimetry in all the radiation fields to
which it may be exposed. There are however some dosimeters that perform well
across the board, and could be used in a wide range of occupations, the MPG2000XB
being one such dosimeter (International Atomic Energy Association, 2007). All of
the active dosimeters used in this thesis are MGP2000 or MGP3000 (the successor
to the MGP2000) models.
1 Introduction 10
There have been some papers reporting direct comparison of active and passive
dosimeters in specific workplaces. In nuclear power production Singh et al. (2013)
found good agreement between Saphydose APDs and TLDs when comparing 29000
results during both normal reactor operation, and during refuelling outages. In
the same paper Singh also outlined controlled experiments which showed good
agreement in results from APDs and TLDs when exposed to known doses from a
Cs-137 source. Other experiments comparing multiple types of APD to TLDs in
controlled conditions (Boziari et al., 2011) have produced less convincing results. The
main conclusion of the Boziari paper was to underline the importance of choosing
the correct APD for your work practices, and understanding any limitations it may
have. No comparisons were found in the literature of passive and active dosimeters
for workers exposed to PET isotopes or other positron emitters. Hence, there is
a need to assess the suitability of using active dosimeters in this type of radiation
exposure environment if we are to consider removing passive dosimeters.
1.3 The Accuracy of Personal Radiation Dosime-
ters
Assumptions are made when using a personal dosimeter to assess the exposure of
a worker. The dosimeter only occupies a small volume in space compared to the
worker, and radiation fields are often inhomogeneous across the worker due to the
effects of geometry, and the presence of shielding materials. It is also assumed
that the dosimeter is worn whenever the worker is occupationally exposed, and not
exposed when the worker is not. Passive dosimetry services rely on the dosimeter
being returned on time, with the appropriate control badge, such that background
radiation levels are subtracted appropriately. If this is not the case an estimate of
the background dose will be subtracted introducing greater error into the results.
The reader will appreciate that where large numbers of people are required to keep
track of small objects over long periods of time some of those objects will become
misplaced either temporarily or permanently. The loss of, and damage to dosimeters
leaves gaps in the data which can generate significant error in the estimation of
personal exposure, significant exposure events could be completely missed from the
record.
The range of results provided by different service providers for dosimeters exposed
in controlled conditions has been evaluated (Böhm et al., 1994), and large differences
were found between providers, and from the expected values. Due to the large errors
1 Introduction 11
inherent in measuring small radiation doses using small dosimeters it should be
noted that results for personal dosimetry are more indicative of personal radiation
exposure than they are an accurate measure of it.
1.4 Active Dosimeters for Legal Assessment of
Occupational Dose
Significant improvements in radiation protection have been obtained through the use
of active personal dosimeters (Bolognese-Milsztajn et al., 2004). Their ability to give
instant feedback allows for radiation workers to adjust their technique while working,
and also give dose information for post work assessments and incident investigations
immediately.
There have been a number of suggestions that active dosimeters will replace
passive dosimeters as the legally accepted means of measuring and recording occu-
pational exposure (Ortega et al., 2001; Luszik-Bhadra et al., 2007). The argument
in favour of APDs is that occupational doses will be reduced through the effect of
instant feedback. With such feedback, workers are more aware of the dose rates they
are exposed to and can adjust their work practices to avoid their highest levels of
exposure. Lower levels of exposure can be reported when using electronic monitors,
as they have a much lower minimum detectable level. An active monitor will display
a dose of a single micro-sievert where passive dosimeters can only report doses above
10, 50 or 100μSv depending on the type. In the event of malfunction or damage to
the active dosimeter, the loss of dose information is reduced, as the dosimeter can
be readily replaced soon after the event. In comparison, a problem with a passive
monitor may not be detected until it is sent for reading at the end of the wear
period. Despite the advantages given, and significant improvements in performance
in the recent past, very few jurisdictions use active dosimeters for legal assessment
of occupational dose (Ginjaume, 2011). The main arguments given against replacing
passive dosimeters are that passive dosimeter’s have a long pedigree of reliable use;
have proven reliability in a wide range of radiation fields, are compact in size and
light weight, and are low cost (Ortega et al., 2001; Luszik-Bhadra et al., 2007).
Before the current working practice can be changed, regulatory bodies must be
convinced that active dosimeters are capable of providing a reliable record of worker
exposure, and any legal hurdles involving the nature of personal dosimetry services
must be overcome.
1 Introduction 12
1.5 PET Radiopharmaceutical Production
At Sir Charles Gairdner Hospital, PET radiopharmaceutical production occurs in
the Radiopharmaceutical Production and Development (RAPID) Laboratories.
PET radiopharmaceuticals have two parts, the positron emitting radioisotope
which can be detected by the PET scanner, and the molecule to which it is attached.
The molecule is chosen as it has a particular behaviour in the body of the patient,
which enables a biological function to be detected or evaluated (Ametamey et al.,
2008). Production of radiopharmaceuticals has two main parts, the production of the
required radioisotope, and the incorporation of the isotope into the pharmaceutical
molecule by a series of chemical reactions.
PET isotopes are produced by proton bombardment of a suitable target material
in a cyclotron. A cyclotron accelerates hydrogen ions using a powerful oscillating
magnetic field to produce a high energy (10-20 MeV) beam of protons. The beam
is incident upon a target containing atoms which absorb the protons and undergo
radioactive decay to form the desired positron emitting isotope. The most widely
used PET isotope is Fluorine-18 (18F) which is produced by proton bombardment
of water enriched with Oxygen-18 (18O) as shown in equation 1.1.
188 O +1
1 p →189 F +1
0 n + ν (1.1)
A number of other reactions can be used to produce 18F using different target
materials and particle beams. The 18O p,n reaction has proven the most cost effective
despite the expense of the target material, due to the relatively low beam energy
required, and the large yields that can be obtained (>100GBq) (Bailey et al., 2015).
In addition to the desired radionuclide, other isotopes can be produced. Protons
in the beam can be absorbed by atoms other than the target atoms and be transmuted
to radioactive species. Also components within the cyclotron can absorb the neutrons
produced in the p,n reaction shown in equation 1.1 and become radioactive. All
of the produced radioisotopes can potentially pose a radiation risk to staff working
with the cyclotron.
Once enough of the desired radioisotope has been produced, the target material
is transported to hot-cells, where the desired isotope is separated from other target
materials. In the case of 18F production, the water target is pumped through shielded
tubing from the cyclotron bunker into the hot-cell. For the production of routine
PET radiopharmaceuticals the chemical separation from the target material and
incorporation into the final molecule is a semi-automated process.
1 Introduction 13
In RAPID, prior to the arrival of the target material in the hot-cell, a kit con-
taining the chemical reagents and any disposable piping, filters, reaction chambers
and vessels are attached to a production system and checked by a radiochemist.
The production system transfers the target material, and the intermediate and final
products, through the various reaction chambers for appropriate amounts of time
and may provide heating to speed up chemical reactions where required (see figure
1.6). The final product is a small volume (~10ml) of very high specific activity18Fluorodioxyglucose (18FDG) in aqueous solution (IBA Molecular, 2010). The final
product is transferred to an automated dispensing unit in a separate hot-cell, which
splits and dilutes the product into multiple doses for delivery to customers, and for
quality assurance testing. The automated process is monitored by the radiochemist
to ensure that all the steps of production are progressing correctly. The radiochemist
is responsible for ensuring the final activities dispensed are suitable for the customers’
needs, and for performing the required quality assurance processes. The radiochemist
then removes the shielded product from the hot-cell and packages it for distribution
to the PET centres. A separate sample may be dispensed for individual doses which
must be drawn up by hand. Drawing up doses by hand from a large activity can
expose the radiochemist to a significant radiation dose, particularly to the hands.
Figure 1.6: Example diagram of an FDG synthesis system (IBA Synthera)
1 Introduction 14
1.6 PET Radiopharmaceutical Dispensing and Use
Once dispatched to the PET centre, the large bolus of 18FDG is transferred to
and loaded into an automated dose dispenser by a nurse or technologist. The dose
dispenser measures the activity of the bolus, which is compared to the expected
activity supplied by the RAPID radiochemist. The dose dispenser can then deliver
individual patient doses via intra-venous lines inserted by nursing staff. Through use
of the automated dose dispenser, staff can be some distance from the line delivering
the patient doses and protected by shielding material from the patient while they
are at their most radioactive. Other radiopharmaceuticals may be delivered as single
doses, in shielded syringes, which are hand injected into the patients. Due to the
need for handling of the dose, exposure of the staff is higher for hand doses.18FDG follows the same metabolic path as glucose, accumulating in cells with
higher metabolic function, such as cancer cells. Concentrations of 18FDG are detected
during the PET scan and can be used to diagnose and track cancer and other diseases.
In order to allow time for bio-distribution and to prevent accumulation in muscle
cells, patients rest between the injection and scanning, typically between 45 and 60
minutes. Immediately after injection the dose rate from the patient is of the order
of 0.092μSv/h/MBq at 1m (Madsen et al., 2006), this means a typical dose rate of
around 23μSv/h at 1m. To minimise radiation exposure to staff and other patients,
the PET patients rest in shielded bays monitored by CCTV. At the end of the rest
period they are escorted to the scanner, positioned and scanned. Scan times vary
depending on the volume of the patient being scanned. The scan rooms are shielded
to reduce dose to the technologists operating the scanner and people in surrounding
rooms, including those above and below the scan room. Due to the penetrating
nature of positron annihilation photons there is no monitoring window between the
control and scan rooms and patients are monitored on the scanner by use of CCTV.
Technologists attempt to minimise contact time with the patients, but it is often
unavoidable, particularly with patients with reduced mobility.
1.7 PET Centre workers
Hospital workers are one of the largest groups of occupationally exposed workers to
ionising radiation (Covens et al., 2007), and medical applications account for the
largest collective dose to radiation workers of any industry (Holmberg et al., 2010).
Positron Emission Tomography (PET) relies on the production of pairs of photons
from positron annihilation. Positrons are emitted in the radioactive decay of an
1 Introduction 15
isotope and rapidly annihilate on contact with an electron in the environment. Each
of the photons produced has at least the energy of half the rest mass of the positron-
electron pair, i.e. 511keV. Positron emitting isotopes may also have alternate decay
modes producing high energy gammas, for example 18F decays 3% of the time by
electron capture producing a 1.66 MeV gamma (Delacroix et al., 2002). Due to
the high energy, and therefore high penetrating power of annihilation and other
photons, the dose minimisation precautions required to protect staff working with
PET isotopes present a special challenge (Madsen et al., 2006), greatly increasing the
requirements for shielding materials compared with other medical imaging modalities.
The RAPID group, consisting of radiochemists and cyclotron engineers are re-
quired to use active personal dosimeters in Western Australia (Radiological Council
of WA, 2010). The cyclotron engineers are responsible for the maintenance of the cy-
clotron, the associated radioactive material transport systems and the hot-cells and
synthesis equipment used in the production of radiopharmaceuticals. They are often
exposed to the radioactive products produced by the cyclotron and synthesis process
and are also exposed to neutron activated components of the cyclotron during regular
maintenance, and when undertaking repairs and upgrades. The radiochemists are
responsible for producing radiopharmaceuticals containing the isotopes produced by
the cyclotron. Even with the automation of much of the chemistry, radiochemists
are still exposed when handling and transporting the shielded doses. Radiochemists
are also exposed when performing quality assurance tests which require handing of
samples. Radiochemists are regularly working with tens of GBq of activity and so
maintenance of good radiation hygiene is vital.
Nursing staff and imaging technicians in Nuclear Medicine are not legally required
to wear active dosimeters, but those who work with PET patients receive higher
occupational doses than others in the same department (Covens et al., 2007). The
dose rates involved are lower than in radiopharmaceutical production, but the staff
can be exposed for significant periods of time when they are in close proximity to
the patients. Even when not close to patients radiation shielding is not capable of
reducing the dose rate to zero, and so staff are exposed to above background levels
of radiation for most of the working day, adding to their cumulative exposure.
RAPID workers are accustomed to wearing APDs, whereas Nuclear Medicine
workers are not. This historical difference in usage offers an opportunity to gather
information on attitudes to the use of active dosimeters from the two groups to
investigate if familiarity affects their opinions. As both groups are routinely exposed
to doses measurable by passive personal dosimeters, they are an ideal population for
1 Introduction 16
a comparison study of the two dose measurement methods (passive and active) in
real workplace environments.
1.8 Aims
It is the intent of this thesis to explore whether the type of logging active personal
dosimetry system, currently used by the staff working in PET radiopharmaceutical
production at Sir Charles Gairdner Hospital, would be suitable as a legal dosimeter,
in place of the currently used passive dosimeters, for workers exposed to PET
radiopharmaceuticals and patients.
There are several criteria that would need to be met for the active dosimeters
to be suitable. They must be shown to have adequate detection capabilities for
the radiation emitted by PET radiopharmaceuticals, either as good as, or better
than the passive dosimeters currently used. They must be shown to be as reliable,
or more reliable than passive dosimeters. These two factors will be examined in
controlled exposure experiments. Dosimeters of different types will be exposed to
known quantities of 18FDG, and their results compared. The doses reported by
passive and active dosimeters worn by staff in RAPID and the PET centre will also
be compared.
Active dosimeters must also be accepted by the staff working with PET radio-
pharmaceuticals, and preferably offer benefit to and be appreciated by the staff. This
will be assessed by obtaining feedback from the staff in the form of a questionnaire
after a period of using both types of dosimeter.
Economic factors must be considered. If active dosimeters are to replace passive
dosimeters in areas where the use of active dosimeters is not mandated they must
be cost competitive with the use of passive dosimeters.
The final consideration is legal. Dosimeters must be approved for use by the
regulatory body of the state in which they are used. At present the active dosimetry
system under consideration in this thesis is not approved. The legal hurdles facing
approval of this types of dosimetry system will be explored.
Chapter 2
Experimental Methods &
Materials
This chapter outlines the materials and methods used to compare the performance of
the active and passive dosimeters. A series of controlled experiments were performed
to directly compare the results from passive and active dosimeters when exposed
to a known quantity of 18FDG. The methods used to compare the results from the
passive and active dosimeters worn by staff working with PET radioisotopes are also
discussed. The section ends with a discussion of the method used to obtain feedback
from staff regarding the use of passive and active dosimeters.
2.1 Radiation Source
All of the radiation sources used in the controlled experiments consisted of a 5ml
glass vial containing less than 1ml of 18FDG in aqueous solution. The activity of
the source at a specific point in time, close to the start of each experiment, was
obtained using a calibrated well counter. The activity of the sources used are given
in tables 2.3 and 2.4. The half-life of 18F is 1.83 hours and the gamma constant for
a glass vial at 1m is 0.158 μSvMBq-1(Delacroix et al., 2002). The gamma constant
is a value derived from computer models of a given isotope in a particular geometry.
The constant gives the dose rate per MBq at 1m. Other references (Madsen et al.,
2006)quote different values for the gamma constant. The value from Delacroix et al.
(2002) was chosen as it is based on the same geometry (a small glass vial) used in
the controlled experiments.
17
2 Experimental Methods & Materials 18
2.2 Passive Dosimeters
In the controlled experiments two types of passive dosimeter are used, TLD badges
(described in section 2.2.1) and OSL badges (described in section 2.2.2). The dosime-
ters are held vertically in a dosimeter holder clipped to a thin piece of ABS (a
common thermoplastic) slotted into holders on a track. The track and the layout
of the controlled experiment are described in section 2.4.2. Dosimeters used in the
experiments were returned directly after exposure along with an unexposed “control”
badge used to remove the contribution from background radiation.
All PET and RAPID workers wore TLDs (described in section 2.2.1) during the
study. Workers are required to wear the dosimeters on their torso whenever working
in an area where they may be exposed to ionising radiation. They are usually worn
clipped at waist or chest height. TLDs are supplied in batches for use over a one
month wear period (for RAPID and PET Centre staff) and then returned to the
supplier for reading.
Both of the passive dosimeter types used are approved as legal personal dosimeters
in Western Australia, and are in use at Sir Charles Gairdner Hospital. They have
been compared to assess the degree of agreement that could be expected between
approved dosimeter types. Because of time and financial constraints it was not
possible to compare all of the approved passive dosimeters with each other. These
two dosimeter types were chosen as they are readily available, in use in the hospital
where the experiments were undertaken, and utilise two different scintillant materials
and reading methodologies.
2.2.1 TLD
The Pansonic UD-802 TLD dosimeters used in the experiments are supplied by
Global Medical Solutions (GMS) in Sydney and analysed by the Radiation Detec-
tion Company in the U.S.. The TLD itself is sealed in a plastic case with a small thin
window which allows the passage of less penetrating radiation, enabling differentia-
tion between shallow (Hp0.07) and deep (Hp10) dose (see figure 2.1a). There are four
lithium borate chips in the dosimeter (figure 2.1b) with different amounts of filtra-
tion to improve photon energy discrimiation and tissue dose estimation (Radiation
Detection Company, 2015).
2 Experimental Methods & Materials 19
(a) External image of TLD showing thin window inupper right
(b) Central component of dosimetershowing 4 TLD chips
Figure 2.1: Radiation Detection Company TLD
A copy of a typical GMS dose report can be seen in Appendix B. The suppliers
claim their TLD badges have a reported minimum detection limit of 50μSv. If the
dose recorded by the dosimeter less the control badge dose is less than this limit the
result on the dose report shows as ’ND’.
2.2.2 OSL
The OSL dosimeters used in the experiments are manufactured and analysed by
Landauer. The crystal itself is sealed in a plastic case with a small thin window which
allows the passage of less penetrating radiation, enabling differentiation between
shallow (Hp0.07) and deep (Hp10) dose (figure 2.2a). The casing holds the integrated
filtration and a thin strip of Al2O3:C (figure 2.2b). A copy of a typical Landauer
(a) External image of OSL show-ing thin window left of centre
(b) Internals of OSL showing filters, hole and grid in the casing,and the Al2O3:C dosimeter
Figure 2.2: Landauer OSL
dose report can be seen in Appendix B. The suppliers claim their OSL badges have
a reported minimum detection limit of 10μSv. If the dose recorded by the dosimeter
minus the control badge dose is lower than this limit, the result on the dose report
shows as ’M’.
2 Experimental Methods & Materials 20
Figure 2.3: Active dosimeters DMC2000S, DMC2000X, DMC2000XB and DMC3000
2.3 Active Dosimetry System
2.3.1 DMC 2000 and DMC 3000
The active dosimeters used by staff and in the controlled experiments were DMC
2000 and DMC 3000 personal dosimeters, manufactured and supplied by MGP
Instruments. Four models of dosimeter were used in the controlled experiments
(see table 2.1 for descriptions) as they are used interchangeably by staff in RAPID.
Staff in Nuclear Medicine prefer the 2000 models as they are physically smaller. In
the controlled experiments the dosimeters were selected at random depending on
availability, reflecting the way they are used by staff. All four models were used. This
decision was validated by the small variability in results between active dosimeters,
shown in figure 3.1 and table 3.6. When in use a small screen displays the dose
accumulated since log in and the dosimeter ’chirps’ when radiation is detected. The
rate of the ’chirps’ provides feedback to the user on the dose rate they are currently
exposed to.
2 Experimental Methods & Materials 21
Model Measures dose from
DMC 2000 S Gamma onlyDMC 2000 X Gamma & x-rays
DMC 2000 XB Gamma, x-rays & betasDMC 3000 Gamma only (new model)
Table 2.1: Active Dosimeter Models Used
The X and XB models of the MGP contain a thin window to allow calculation
of shallow (Hp0.07) as well as deep (Hp10) dose. In PET the radiation risk comes
from penetrating gammas rather than low energy x-rays or betas, so only the deep
(Hp10) dose is of concern. The dose measurements compared in this study are the
deep (Hp10) doses from the active and passive dosimeters, all of the models of APD
measure deep dose, so they can all be used. Different dosimeters were used in each
controlled experiment as this reflects the way the dosimeters are used by staff. The
IDs of dosimeters used in the controlled experiments were recorded so that any
differences in results between the active dosimeters could be investigated if any were
found.
2.3.2 Logging Station & Database
A dosimeter logging station allows radiation workers to assign a dosimeter to them-
selves before beginning their shift, and log the dosimeter out at the end. At log
out the logging station receives the dose information, displays it to the worker, and
transmits the data to a database for storage. The stations consist of networked
touch screen PCs running the LDM MGR software with a USB dongle or cradle
which enables wireless communication with the MGP dosimeters.
At the start of their shift workers place the dosimeter in the cradle, and enter
their unique identifier code on the touchscreen. The logging station assigns the
dosimeter to that worker, setting the appropriate dose and dose rate alarm levels.
The alarm levels are chosen such that they will not alarm during routine procedures,
only in the event of an unexpectedly high dose or dose rate. As different dose rates
are experienced by different occupations the levels are assigned by occupation.
2 Experimental Methods & Materials 22
Figure 2.4: Logging Station with dosimeter in cradle
Dose (μSv) Dose Rate (mSv/h)
Visitor 20 0.1Nurse/Technologist 100 10
Radiochemist 80 10Engineer 200 12
Table 2.2: Alarm Settings on Active Dosimeters
At the end of their shift the worker returns the dosimeter to the cradle, and the
dose data is read from the dosimeter and saved to the database. The total dose
and maximum dose rate per day for the last few days logged in are displayed on
the screen, allowing the worker to quickly review their dose, and check for unusual
readings.
The logging stations read and write data from a relational database running on
a networked PC. This PC also runs the Dosicare software which is used as a front
end to the database. The Dosicare software can be used to add new users, create
or modify user group profiles (with associated dose and dose rate alarm levels), and
generate dose reports.
2 Experimental Methods & Materials 23
Figure 2.5: Logging Station showing dose results at log out
2.3.3 Software
The MGP active dosimetery system requires a number of software components to
function. LGM MGR is run on the logging stations, it handles logging dosimeters in
and out of the system. Dosicare is used to modify user and dosimeter information
in the database and to configure and produce reports. It can also be configured to
send email alerts if recorded doses exceed threshold values. All of the MGP software
communicates with a Microsoft SQL Server Express installation which holds the
database of user, dosimeter, and dose information.
2 Experimental Methods & Materials 24
2.4 Controlled performance comparison of passive
and active dosimeters
In order to judge the suitability of active dosimeters as a legal dosimeter in PET
applications, one of the first considerations is their ability to accurately record
radiation dose when exposed to radiation from a PET isotope. There are a number
of different providers of passive dosimeters that are approved for use in Western
Australia (Radiological Council of WA, 2010). The methods below explain how a
comparison was made between the results from active dosimeters and two types of
approved passive dosimeters. It is reasoned that if the difference in results between
the active and passive dosimeters is less than, or similar to, the difference between
the results from the two types of passive dosimeters, then the active dosimeters
can be said to be equivalent in terms of accuracy of measurement. The dosimeters
used were chosen because the passive dosimeters are both approved for use as legal
personal dosimeters in Western Australia; the active dosimetry system has been in
use for a number of years in the hospital and is capable of automatically producing
a record of each individual’s exposure.
A controlled experiment was undertaken in order to remove the variations which
occur from person to person and day to day and concentrate simply on the ability of
the dosimeters to measure radiation doses from a PET radiopharmaceutical across the
range of doses typically received by radiation workers in PET radiopharmaceutical
production and use. The controlled experiments consisted of a number of planned
simultaneous exposures of dosimeters to the radiation from a vial containing18FDG
the most commonly used PET radiopharmaceutical.
2.4.1 Radiation Safety
The experiments require the use of an unshielded source of penetrating ionising
radiation (511keV gammas). It is essential to ensure that the experiment is conducted
in a controlled area and that dose rates in any surrounding uncontrolled areas do
not pose a risk to anyone. The controlled area has restricted access with signage
and a physical barrier at the single access point during the period of the exposure.
Dose rate measurements were taken in accessible areas around the controlled area
to ensure that dose rates were below 25μSv/h. All areas surrounding the controlled
area were low occupancy areas (corridors and stairwells). The limited number of
exposures carried out also reduced the risk of cumulative doses to staff from the
experiments.
2 Experimental Methods & Materials 25
Lead shielding was placed such that the source could be approached from the
entrance to the controlled area with minimal exposure to the experimenter (Figure
2.6).
2.4.2 Physical layout of experiment
At least three of all three types of dosimeter were arranged simultaneously at a fixed
distance of 1m from the vial of FDG in an arc by means of positioning them on
stands on a circular rail (Figure 2.6) for each exposure. The circular nature of the
apparatus and the central source ensures that the dose to each dosimeter was equal
including the possible effect of scatter from the floor.
Figure 2.6: Passive dosimeters arranged on 1m radius rail
The different types of dosimeter (figure 2.7) were placed alternately around the
arc of the rail. The spread of each dosimeter type around the rig removes any
possibility of systematic bias between dosimeter types caused by geometry.
2 Experimental Methods & Materials 26
Figure 2.7: Personal dosimeters on holders.
From left to right: an OSL passive dosimeter, an active MGP DMC2000X dosimeterand a TLD passive dosimeter.
The dosimeter holders contain slots which allowed them to be positioned on
the rail, and to hold the dosimeters parallel with the rail or at and angle of 30° or
60° from the rail (figure 2.8). These slots allowed for accurate positioning of the
dosimeters when comparing the effect of angling the dosimeters relative to the source
of radiation. In a real world application, a source will not always be perpendicular
to a detector, so a dosimeter should still give a reasonable measurement when angled
away from the source. 30° and 60° were chosen to match the methodology used
by the IAEA when comparing multiple types of active dosimeter, including one of
the models of MGP dosimeter used here (International Atomic Energy Association,
2007). In each experiment at least three dosimeters of each type were irradiated
simultaneously. All of the dosimeters in each experiment were at either 0, 30 or
60 degrees from perpendicular to the radiation source. All dosimeters irradiated
simultaneously were at the same angle to the source. Simultaneous irradiation at
the same distance from the same source allows for direct comparison of the results
from the dosimeters in each experiment. The methodology for comparing between
exposures is explained in section 2.4.6.
2 Experimental Methods & Materials 27
Figure 2.8: Dosimeter holders on the rail at 0, 30 and 60 degrees
As the active dosimeters contain metal components it was thought that there may
be a small amount of scatter from the active dosimeters. To remove the possibility of
scatter affecting adjacent dosimeters the dosimeters were placed alternately by type
on the holders with a minimum of 10cm space between each one. A minimum of
three of each type of dosimeter were placed on holders around the ring to negate any
unknown geometric effect when comparing between dosimeter types. An example
layout showing dosimeter placement is shown in figure 2.9.
2 Experimental Methods & Materials 28
Figure 2.9: Example plan layout of experimental setup
Figure not to scale
To ensure that there was no geometric bias between the positions around the rig
the mean of the reported dose at positions A, B and C in each exposure for each
dosimeter type was calculated. The percentage difference from these means for each
dosimeter type was calculated for each usable result. The average of these percentage
differences for positions A, B and C were then calculated across all dosimeter types.
The results are shown in table 3.3.
2.4.3 Conducting an Exposure
With the dosimeters in place, a shielded vial containing a known activity of 18FDG
was placed in the centre of the apparatus and the shielding removed. The activity
2 Experimental Methods & Materials 29
and exposure time can be chosen to deliver a given exposure to the dosimeters using
DT =T1/2ΓA0
ln2
(
1 − e−T ln2
T1/2
)
(2.1)
Where DT is the predicted effective dose to the dosimeter, T1/2 is the half-life of
the isotope, Γ is the gamma constant for the isotope, A0 is the activity of the source
at the start of the exposure and T is the length of the exposure. This equation
accounts for the decay of the isotope during the exposure which is significant for
PET isotopes as they typically have short half-lives. The values for T1/2 and Γ for
the sources used were given in section 2.1.
2.4.4 Exposures Performed
Personal dosimeters must be shown to work accurately over the range of exposure
levels and dose rates experienced by staff, thus a series of exposures were carried out
with different activities of 18FDG. In order to investigate the angular dependence
of the dosimeters exposures were repeated with the dosimeters rotated through
30 and 60 degrees following the methodology of the IAEA (International Atomic
Energy Association 2007). Initial results showed no change in dose rate related
to position around the experimental setup. This meant that there was no need to
repeat exposures with varied positions around the rail.
Tables 2.3 and 2.4 show the exposures performed on dosimeters giving an esti-
mation of the dose to the dosimeters based on equation 2.1. The low doses were
intended to investigate the claimed minimum detectable level for the passive dosime-
ters, in addition to comparing their results with the active dosimeters. The largest
doses were delivered using three separate exposures of the same passive dosimeters
in order to keep the dose rate outside the controlled area to a reasonable level. A
larger number of active dosimeters were used when they were available to increase
the number of points of comparison produced by each exposure.
Some of the OSL dosimeters returned lower than expected results requiring a
repeat of some of the exposures.
2 Experimental Methods & Materials 30
Initial Source Exposure Degrees Calculated Number ofExperiment Activity Duration from Dose Dosimeters
(MBq) (hours) perpendicular (μSv) OSLs TLDs MGPs
1 101 2.60 0 25 3 3 32 254 1.00 30 31 3 3 33 253 1.25 60 37 3 3 34 698 1.50 0 117 3 3 35 693 1.48 30 100 3 3 36 708 1.22 60 102 3 3 57 1101 2.55 0 246 3 3 48 1005 2.88 30 261 3 3 49 1010 3.10 60 268 3 3 4
10a 1011 2.800 762 3 3
410b 1009 2.95 410c 1043 2.50 4
Table 2.3: Exposures performed in initial controlled experiments
Initial Source Exposure Degrees Calculated Number ofExperiment Activity Duration from Dose Dosimeters
(MBq) (hours) perpendicular (μSv) OSLs TLDs MGPs
11a 986 2.870 753 3 3
411b 1025 2.67 411c 1012 2.75 4
Table 2.4: Exposures performed in repeated controlled experiments
2.4.5 Obtaining Results
In the controlled experiments dosimeters were logged in using guest log-in codes and
the doses recorded manually at log out for each dosimeter after each experiment .
The passive badges were returned to the companies that supplied them within a few
days of exposure to be read and reported on. Dose reports were returned within a
few months and the results compared to those from the active monitors. The result
from each dosimeter was compared to the results from each of an alternate dosimeter
type from the same exposure. Each comparison provides a point on the Bland-
Altman plots shown in the Results section, starting on page 37. It was expected
that the results from the same dosimeter type in each experiment would be similar.
Comparisons of results from the same dosimeter type in each experiment were made
using the same methodology as the inter-type comparisons. The result from each
dosimeter was compared to the results from the others in the same experiment. Each
result compared had the dosimeter exposed to the same source, for the same amount
2 Experimental Methods & Materials 31
of time, at the same angle to the source; the only difference being their location
around the experimental rail. Any significant differences between results from the
same dosimeter type would reveal problems either with the dosimeters themselves,
or with the experimental set up. The results from the intra-type comparisons can be
seen in section 3.2.2. There were some failures of passive dosimeters, these failures
are discussed in chapter 3. Some results were excluded from comparison as they
were obviously in error and would skew the results. Dosimeters which reported the
dose as “below the detectable limit” when the other results for the same exposure
were greater than 20% over the reported detection limit were excluded. The number
and type of excluded dosimeter results are recorded in table 3.2 on page 38.
2.4.6 Normalising results from separate exposures
It was not possible to expose multiple dosimeters of each type at all three angular
positions simultaneously. As shown in table 2.3, the experiments with the dosimeters
0°, 30° and 60° from perpendicular to the source were separate. Each experiment
involved different activities of 18FDG and different lengths of time and thus the
measurements from the separate experiments cannot be directly compared. The
theoretical dose to the dosimeters (see equation 2.1) takes into account the activity
and exposure time for each experiment. The theoretical dose can therefore be used
to normalise the results for comparison of results from separate experiments.
The scaled result from the 30° experiment (Ds30) is calculated from the actual
result from the 30° experiment (D30) scaled by the ratio of the theoretical dose in
the 0° experiment to the theoretical dose in the angled experiment DT 0
DT 30
.
Ds30 = D30DT 0
DT 30
(2.2)
The results of equation 2.2 can now be compared to the results from the 0°
experiment in a Bland-Altman plot. Plots have been generated comparing the
results at 0° with 30° and 0° with 60° for each dosimeter type. The results can be
seen in section 3.2.4.
2.4.7 Displaying Results
The majority of results in Chapter 3 are displayed as plots. One type of plot shows a
direct comparison of results from two dosimeter types, with the dose result from each
dosimeter type forming the axes of the plot. The direct comparison plots include a
least-squares fit to the data. Bland-Altman plots are also used extensively.
2 Experimental Methods & Materials 32
The direct comparison plots have the reported dose from one dosimeter type on
the x axis, and the results from the other on the y axis. If the two sets of results
agreed perfectly the least squares linear fit to the points on the graph would be the
line of x = y with a correlation coefficient (r2) of one. The further the linear fit is
from x = y, and the lower the correlation coefficient, the worse the agreement is
between the dosimeter types.
Bland-Altman plots are used to analyse the agreement between two different
measurement methods of the same variable (Bland and Altman, 1999). Each point
in a plot is a comparison between two measurement methods of the same variable,
in the case of these experiments, radiation dose. The point’s position along the
x axis is the average of the results of the two measurements, and the position on
the y axis is the difference between the results. If the two measurement methods
were perfectly correlated, all the points would lie along the line of y=0. A constant
systematic bias can be seen as a shifting of the points up or down away from y=0. If
the points in a Bland-Altman plot slope away from the x-axis, this indicates a need
for a multiplicative correction factor to correlate the results.
In the controlled experiments multiple dosimeters of each type were used. As they
were all exposed to the same radiation they can all be said to be making the same
measurement. This provides multiple points of comparison for each result and there-
fore multiple points in each Bland-Altman plot. As an example, a single exposure of
3 dosimeters of type A, 3 dosimeters of type B, and 3 dosimeters of type C (as shown
in figure 2.9), produce 27 comparison points (A1-B1,A1-B2,A1-B3,A2-B1,A2-B2,A2-
B3,A3-B1,A3-B2,A3-B3, A1-C1,A1-C2,A1-C3,A2-C1,A2-C2,A2-C3,A3-C1,A3-C2,A3-C3,
B1-C1,B1-C2,B1-C3,B2-C1,B2-C2,B2-C3,B3-C1,B3-C2,B3-C3) which would be shown
as 9 points in each of three plots, one comparing A to B, one comparing A to C,
and one comparing B to C. The equivalence of positions 1 to 3 were evaluated by
calculating the difference between the mean result for all three positions with the
result from each position for each dosimeter type.
The Bland-Altman plots in Chapter 3 include lines showing the mean difference
between the results, which demonstrates any systematic bias between the measure-
ment methods. An indication of the spread of results is shown by the ±1.96σv (95%
confidence interval) lines.
In addition to the detailed comparison plots, tables summarising the results and
plots displaying the aggregated data can be found in section 3.2.8.
2 Experimental Methods & Materials 33
2.4.8 Statistical Assessment of Difference of Means
For each pair of dosimeter types (active-TLD, active-OSL and TLD-OSL) a test of
the difference of the means was performed. The results of experiments at similar
exposure levels were grouped by normalising the results from the 30° and 60° to the
perpendicular results as per section 2.4.6. The mean and standard deviation of each
set of results was calculated, and from these the standard error of the difference of
the means and the degrees of freedom. As the standard deviations for each set of
results were not similar, equations 2.3 and 2.4 were used.
Standard Error:
SE[X̄A − X̄B] =
√
s2A
nA
+s2
B
nB
(2.3)
Degrees of freedom:
df =[
s2
A
nA+
s2
B
nB]2
[s4
A
n2
A(nA−1)+
s4
B
n2
B(nB−1)]
(2.4)
The test statistic T was calculated using equation 2.5 and compared to the t
distribution to judge agreement between the means.
T =(X̄A − X̄B)
SE[X̄A − X̄B](2.5)
The results of the tests are shown in section 3.2.9.
2 Experimental Methods & Materials 34
2.5 Comparison of staff doses recorded by passive
and active dosimeters
2.5.1 Gathering RAPID Staff doses
Radiochemists and Engineers working in radiopharmaceutical production and devel-
opment (RAPID) at Sir Charles Gairdner Hospital have been wearing both passive
and active dosimeters since the inception of the service in 2003. In 2012 a logging
system was introduced which allowed automatic recording of the results from the
active dosimeters to a central SQL database. It is a requirement of the Radiological
Council that all staff and visitors entering the RAPID area must wear an active
dosimeter. All staff are also legally required to wear an approved passive dosimeter
whenever they are working with radioactive material, or are in an area where they
may be occupationally exposed to ionising radiation. As the staff are wearing both
dosimeters at all times, a comparison of the total monthly whole body exposure
measurements from the two dosimeter types was performed. Staff doses from the
active dosimetry system were extracted from the SQL database with a query which
summed doses over a month for each worker. The monthly doses were copied into a
spreadsheet for comparison with the doses reported by the passive monitors.
The number of staff has varied from year to year, and not all staff have worked
in RAPID every month in a year (table 2.5).
Type of Worker Number of staff (and months of data) in RAPID2012 2013 2014
Radiochemist 5 (53) 5 (59) 5 (59)Engineer 3 (36) 3 (35) 3(36)
Research Chemist 3 (29) 5 (48) 3 (34)Total 10 (118) 13 (142) 11 (129)
Table 2.5: RAPID Staff Numbers 2012-2014
2.5.2 Gathering PET Centre Staff doses
The PET Centre staff did not routinely use active dosimeters prior to commencement
of this project. The PET Centre is within the Nuclear Medicine department, and
the majority of nursing and medical imaging staff who have days working with PET
patients also work some days in Nuclear Medicine. These staff will receive some
exposure from non-PET patients. Due to a limited budget, there were not enough
2 Experimental Methods & Materials 35
dosimeters to track all the staff who work with PET patients while they work in
PET and other areas of the Nuclear Medicine department.
It was originally thought that the doses from PET patients would constitute
the majority of the exposure such that the results from the passive badges, and the
active badges should show correlation even if the active dosimeters were only worn
by the staff on the days they were working in PET. It was thought that including a
larger number of staff (table 2.6) would make for more useful data.
Profession Number of staff
Nurse 11Technologist 5
Table 2.6: Numbers of staff wearing active dosimeters when working in PET only(Nov 2012 to December 2013)
The correlation of active and passive dosimetry results from 2013 proved to be
very poor (see figure 3.18) with the passive dosimeters consistently reporting higher
doses. In 2014 specific staff members were asked to wear the active dosimeters
in PET and Nuclear Medicine such that they were always wearing passive and
active radiation monitors when working. The staff chosen were the ones who had
received the highest doses recorded by their passive dosimeters in 2013. Seven active
dosimeters were reserved for use by these workers (table 2.7). Other staff could use
any unused active dosimeters when they were available.
Profession Number of staff
Nurse 5Technologist 2
Table 2.7: Numbers of staff wearing active dosimeters while working with PET andNuclear Medicine patients (January to December 2014)
The doses recorded by the active dosimeters were extracted from the database
and compared with the results from the passive monitors, in the same manner as
the results from the RAPID staff.
2.5.3 Comparison of doses
As with the controlled experiments, the results from the passive and active dosimeters
were compared using least-squares fits to a linear relationship, and Bland-Altman
plots. Each point on both types of plot show the comparison of the results from the
active and passive dosimeters for a particular staff member for a single month.
2 Experimental Methods & Materials 36
As with the controlled experiments, if the dosimeters are equivalent the least-
squares fits should be close to y = x, and well correlated with the data. Good
correlation would be demonstrated by a correlation coefficient (r2) close to one.
In the Bland-Alman plots the x axis position of each point shows the average
of the results from the two measurement methods. The y axis position shows the
difference between the active and passive measurement results. For two sets of ideal
measurements all the data points would lie along the line y=0. The Bland-Altman
plots include horizontal lines showing the mean of the difference between the two
measurement methods, and the mean plus and minus 1.96 standard deviations (the
95% confidence interval).
In addition to the standard Bland-Altman plots the staff data is also compared
in Bland-Altman ratio plots. In these plots the y axis value is the result from the
active dosimeter divided by the passive dosimeter result. These ratio plots are useful
when the data in the standard Bland-Altman plot does not follow a horizontal trend.
If there is a systematic bias between two measurement methods, the mean ratio
of one result to the other will show this bias, and the spread of results show how
consistent this bias is. For two equivalent measurement methods the plot should
have points close to y = 1 across the range of values measured.
2.6 User Experience Survey
Staff in RAPID and Nuclear Medicine who have been using both types of personal
radiation monitor were asked to complete a survey (Appendix A). The intent of the
survey was to indicate the level of acceptance of the use of active personal dosimeters,
any preferences between the two types of dosimeter, and the extent to which the
staff felt that either provided a benefit to their radiation safety. The results of the
survey are shown in Section 3.4.
Chapter 3
Results
3.1 Results below the detection limit
As discussed in section 1.2.1 all passive dosimeters have a minimum detection limit.
Suppliers of passive dosimeters do not report doses below this limit, giving the result
as “below the detectable limit”. Some of the controlled experiments exposed the
dosimeters to doses close to the stated detection limit in order to investigate how
the different dosimeter types performed near the limit, and how accurate the stated
limits where.
Minimum detection limit (μSv)
TLD 50OSL 10
Table 3.1: Reported Minimum Detection Limits for Passive dosimeters as stated bysuppliers
In the controlled experiments all results which came back as “below the detectable
limit” are excluded from comparison with the other dosimeter results. In some cases
the results from passive dosimeters came back as “below the detectable limit” when
the results from the other dosimeters, and calculation of the expected dose indicated
the dosimeter was exposed to a dose in excess of the stated minimum detection limit
for the dosimeter type (see table 3.1). The number of results which could not be
used despite an apparent exposure above the stated minimum detection limit are
given in table 3.2.
37
3 Results 38
Excluded Unreliable Results Unreliable resultsresults* (as % of dosimeters of this type used) above 50μSv
TLD 1 1 (3%) 1 (3%)OSL 8 11 (33%) 6 (18%)
Table 3.2: Number of excluded dosimeter results in the controlled experiments
*Badges reporting results below the detection limit when active dosimeterreported doses greater than the stated detection limit + 20%
In the first high dose experiment (3 exposures totalling around 1mSv) the results
of the three OSLs were significantly lower than the results of the other dosimeters,
and the value expected by calculation. These values were not excluded and so are
not included in the first column of table 3.2, but are included in the “Unreliable
Results” columns of the table, taking the total to 11 (6 above 50μSv). The results
of this first high dose exposure can be seen in figure 3.9.
The high dose experiment (10 in table 2.3) was repeated, and the results of the
OSLs from the repeat experiment (11 in table 2.4) were used for all comparisons
shown after figure 3.10. The active dosimeter and TLD results from experiment 10
are still included in comparisons.
Table 3.2 does include three OSLs, used in experiment 9, which reported “below
the detectable limit” when the active dosimeters and TLDs gave results of the order
of 300μSv. The one TLD in table 3.2 reported “below the detectable limit” when the
other two TLDs from the same batch reported 100μSv and 130μSv from experiment
6. There were nine other TLDs that reported “below the detectable limit”. The
expected dose for these nine dosimeters was below or around 50μSv according to
the other dosimeters. These results are not included in the table as the dosimeters
were not expected to report doses at that level. There is no way to compare the
TLD results with those of the other dosimeters in the plots and tables of section 3.2
where the dose is close to or below the reported detection limit.
In the comparison of passive and active dosimeter results for staff there is no
way to know whether the active or passive result is more accurate as there is no
third measurement. In the comparisons of results from staff dosimeters the results
of “below the detectable limit” (BDL) were included as an estimate in the plots
but not in the calculation of the mean difference and the standard deviation. As
the stated minimum detection limit of the used TLDs is 50μSv a value of half the
detection limit (25μSv) was chosen as the estimate for all results returned as “below
the detectable limit” in the plots. To explore the effect of the BDL results, the mean
difference and standard deviations were calculated setting the BDL dose values to
3 Results 39
0μSv, 25μSv and 50μSv and these results are tabulated beneath the comparison
plots. The only months where data were excluded from the plots were those where
the active dosimeters had not been logged in, and the TLD results were “below the
detectable limit”.
3.2 Controlled performance comparison of passive
and active dosimeters
Comparisons of the performance of the dosimeters have been performed by producing
Bland-Altman plots for pairs of dosimeter types as explained in section 2.4.7. The
sections of this chapter demonstrate the variation in results between dosimeters of
the same type to show the repeatability of each dosimeter type. This is followed by a
comparison of results from each dosimeter type with the theoretical dose. The effect
of angulation of each dosimeter type is then explored. The inter-type comparisons
start with the dose results from the two types of passive dosimeter, then the reported
doses from each of the passive dosimeters are compared with the results from the
active dosimeters.
3.2.1 Comparison of doses around the experimental rig
The means of the differences between the results across all positions and the individ-
ual positions were less than 1.5% (table 3.3). This variation is much smaller than
variations in results found within and between dosimeter types in the subsequent
sections of this chapter.
Position A Position B Position C
Mean difference from-1.4% -0.3% 1.4%mean result across positions
for all dosimeter types
Table 3.3: Position dependence of results
3.2.2 Comparison of results from the same dosimeter type
Any repeated experiments, even with the same equipment will show a spread of
results, the smaller the spread the more repeatable a measurement method is said
to be. The following plots compare the results from the same dosimeter type in
each experiment with each other, comparing the spread of those results with the
3 Results 40
spread between dosimeter types. The direct comparison plots are of no use for these
comparisons as the slope of the plot would always be close to unity. As stated
by Bland and Altman (1999) we would not expect a bias between measurements
of the same type but the size of the standard deviation gives an indication of the
repeatability of each dosimeter type.
(a) Bland-Altman plot of OSL results (b) Bland-Altman plot of TLD results
(c) Bland-Altman plot of MGP results
Figure 3.1: Comparison plots for results from the same dosimeter type
For the OSL dosimeters the mean difference is 3μSv, and the 1.96σv value is 92μSv.
For the TLDs the mean difference is 9μSv, and the 1.96σv value is 109μSv, and for
the active dosimeters the mean difference is 4μSv, and the 1.96σv value is 41μSv.
As expected, the mean difference is very low for all three types of dosimeter. It
3 Results 41
is not statistically different from 0 for any of the dosimeters, according to a standard
one sample t-test with a 95% confidence interval. The active dosimeters show a
much smaller spread of results than the passive dosimeters, demonstrating better
repeatability. The OSL results are in close agreement below 500μSv, but the large
spread of results at around 1mSv has greatly increased the 1.96σv value, as shown in
figure 3.1a. The results from the TLDs gradually get further apart with increasing
dose, resulting in the spreading of points away from y=0 in figure 3.1b. The spread
of results for the MGPs is noticeably smaller, resulting in the lower 1.96σv value.
The agreements between the active dosimeters and each of the passive dosimeters
are similar to the level of agreement within the passive dosimeters. These results
again suggests that the active dosimeters are at least as capable of providing accurate
measurements of radiation dose from 18FDG as the passive dosimeters.
3.2.3 Comparison of dosimeter results with theoretical dose
As shown in section 2.4.3; with a known activity at a fixed distance for a known
period of time it is simple to calculate a theoretical estimate of the dose expected to
be received by the dosimeters using the gamma constant for 18F. This calculation is
not expected to be accurate as it assumes a geometry which is only an estimate, and
that no radiation scattered from other materials reach the detector. As the source
and the dosimeters are close to a concrete floor there is expected to be a scatter
contribution. This contribution should be uniform across the dosimeters due to the
design of the experimental rig.
As with the comparisons between different dosimeter types, direct comparison
plots and Bland-Altman plots have been produced. The plots compare the results
from the three dosimeter types to the calculated theoretical dose.
The active dosimeter results show an excellent fit to a linear model (figure
3.2a) but the dosimeter results exceed the theory by 32%. The sets of points form
vertical lines as all the dosimeter results are compared to the same theoretical result
for each experiment. There were two experiments performed with the dosimeters
perpendicular to the source at the high dose level hence the two lines of data.
The Bland-Altman plot (figure 3.2b) shows the mean difference is -60μSv, and the
1.96σv value is 135μSv. The increasing difference in this plot echoes the substantial
difference between the slope of the previous plot and the ideal agreement of x = y.
3 Results 42
(a) Direct comparison (b) Bland-Altman plot
Figure 3.2: Comparison of Active dosimeter results with theoretical dose
(a) Direct comparison (b) Bland-Altman plot
Figure 3.3: Comparison of OSL dosimeter results with theoretical dose
For the OSL dosimeters there is also a good agreement with a linear model (figure
3.3a) but an even higher difference between the reported dose and the theoretical
dose (~50%). In this case the fit does not have an intercept very close to zero. The
higher slope and the negative intercept suggests that the reported high dose results
may be in excess of the actual dose as suggested by earlier comparisons between
dosimeter types. In the Bland-Altman plot (figure 3.3b) the mean difference is
3 Results 43
-50μSv, and the 1.96σv value is 159μSv.
(a) Direct comparison (b) Bland-Altman plot
Figure 3.4: Comparison of TLD results with theoretical dose
For the TLD results the direct comparison plot (figure 3.4a) shows good agreement
with a linear model but an underestimation of the dose by theory (or an over-
reporting by the TLDs) of 29%. The intercept is closer to zero than for the OSL
dosimeters but not as close as for the active dosimeters. In the Bland-Altman plot
(figure 3.4b) the mean difference is -85μSv, and the 1.96σv value is 172μSv.
For all of the theory comparisons the measured dose was significantly higher than
the theoretical dose. In all three cases the mean difference was significantly different
to 0, according to a standard one sample t-test with a 95% confidence interval. In
the direct comparison the slope of the results with increasing dose around 1.3 for
the MGPs and TLDs, and 1.5 for the OSLs, suggests either a significant under
measurement of dose at 511keV for all the dosimeters or a scatter component of the
dose rate of around 30%.
3.2.4 Effects of angling the dosimeters
In real life exposure situations the dosimeter is not always perfectly perpendicular
to the source of radiation. It is important that personal dosimeters can accurately
record dose from a range of angles. When testing dosimeters the IAEA exposes
dosimeters at angles of 30 and 60 degrees from perpendicular to judge their ability
to cope with angulation (International Atomic Energy Association, 2007). The
3 Results 44
results of a similar comparison are presented in this section for the three dosimeter
types assessed.
(a) Bland-Altman plot comparing 0° and 30° re-sults
(b) Bland-Altman plot comparing 0° and 60° re-sults
Figure 3.5: Plots showing the effect of angulation on MGP dosimeters
For the active dosimeters at 30° the mean difference is 3μSv, and the 1.96σv value
is 36μSv (figure 3.5a). At 60° the mean difference increases to 11μSv, and the 1.96σv
value is 44μSv (figure 3.5b). The measured values decrease slightly with angulation.
The difference between 0 and 30 degree results is not statistically significant but the
difference in the mean results between 0 and 60 degrees is statistically significant
according to a standard one sample t-test with a 95% tolerance interval.
For the OSL dosimeters at 30° the mean difference is -31μSv, and the 1.96σv
value is 56μSv (figure 3.6a). This is a larger drop and wider spread than for the
active dosimeters at 30° or even 60°. A batch of three OSL dosimeters failed to
report a dose when measuring the doses at 60°; this greatly reduced the data for
the plot (figure 3.5b), but the mean, and in fact all the results at 60°, are lower
than those at 0°, as expected. The difference between 0 and 30 degree and 0 and 60
degree is statistically significant according to a standard one sample t-test with a
95% tolerance interval.
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(a) Bland-Altman plot comparing 0° and 30° re-sults
(b) Bland-Altman plot comparing 0° and 60° re-sults
Figure 3.6: Plots showing the effect of angulation on OSL dosimeters
(a) Bland-Altman plot comparing 0° and 30° re-sults
(b) Bland-Altman plot comparing 0° and 60° re-sults
Figure 3.7: Plots showing the effect of angulation on TLDs
For the TLDs at 30° the mean difference is 17μSv, and the 1.96σv value is 73μSv
(figure 3.7a). A slightly larger drop, and a wider spread than for the active dosimeters
at 60°. At 60° the mean difference is 55μSv, and the 1.96σv value is 72μSv (figure 3.7b).
The difference between 0 and 30 degree results is not statistically significant but the
difference in the mean results between 0 and 60 degrees is statistically significant
3 Results 46
according to a standard one sample t-test with a 95% tolerance interval.
Mean Difference from 0° (μSv)30° 60°
MGP -3 -11
OSL -31 -53
TLD -17 -55
Table 3.4: Summary of changes in dose readings when angling dosimeters
Figure 3.8: Plot of Normalised Mean Results against angle
Figure 3.8 shows the normalised, mean reported dose for each dosimeter type at
zero, thirty and sixty degrees. Lines of best fit for each type at the two exposure
levels were calculated and included in the plot. Overall there is a slight reduction in
measured dose when angling the dosimeters away from perpendicular to the source
of radiation in all the dosimeters. This is shown by the negative value of the mean
difference in all the Bland-Altman plots and the values in table 3.4. As one might
expect, the effect is stronger overall in all three cases with increasing angle. This
can been seen in the downward slope of the lines of best fit in figure 3.8. The active
dosimeters appear less effected by the angle of incidence than the passive dosimeters.
The results suggest that the active dosimeters would be at least as suitable for
personal dose measurement as the approved passive dosimeters in this regard.
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3.2.5 Comparison of passive dosimeters
The plots in sections 3.2.5, 3.2.6 and 3.2.7 include results at 0, 30 and 60 degrees
from perpendicular to the source. All comparisons are like for like, only dosimeters
from the same exposure, and therefore at the same angle, are compared with each
other. The points are plotted in separate series for the 0, 30 and 60 degree exposures
in all of the plots.
(a) Direct comparison of dose results (b) Bland-Altman Plot
Figure 3.9: Initial comparison of passive dosimeter results
It is obvious in figure 3.9 that there is a problem with the results from the OSL
dosimeters in the high dose experiment. The source of this error is unknown. No
fit was made to the data in figure 3.9a as it is clearly not a linear relationship. All
of the data in subsequent plots for the controlled experiment show good agreement
with a linear relationship fitted to the data as would be expected.
The points in the plot form groups of rectangles because the points all come
from comparing each result from one dosimeter type to all the results from the other
dosimeter type in the same experiment. The width of each rectangle shows the
spread of results for the dosimeter type on the x axis; the height shows the spread
of results for the dosimeter on the y axis in each experiment. A similar effect can be
seen in the Bland-Altman plots that follow, the points from each set of comparisons
form parallelograms. The y value of point of the parallelogram furthest from y=0
shows the maximum difference between the two dosimeter types being compared
in that experiment, the y value of the point closest to the x axis (y=0) shows the
smallest difference between the dosimeter types.
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There are more points in the plot than can actually be seen, as the results from
the passive dosimeters are often the same. They coincide as results from passive
dosimetry services are given to the nearest 10μSv. Coincidence of data is much
less likely for the active dosimeters, as the results are given to the nearest 1μSv.
In the plots with linear fits to the data, all points are used in the least squares fit
calculation, independent of whether they can be seen visually in the plot.
The reported doses for the OSLs exposed to the highest dose were considerably
lower than expected from theory, or recorded by the TLDs or active dosimeters. Due
to this discrepancy the exposures were repeated with new TLDs and OSLs in order
to see if this was a flaw in the reporting of a single batch or a consistent problem with
the OSLs. The repeated experiment gave the results in figure 3.10. In the repeated
experiment, the results of the OSLs where much more in line with the results from
the other dosimeter types. It was concluded that there was an error with the batch
of OSLs used in the first set of exposures, or the reading of them. From this point
all comparisons with OSL results in controlled experiments include the results from
this second experiment rather than the first set of exposures.
(a) Direct comparison with least squares fit (b) Bland-Altman plot
Figure 3.10: Comparison of OSL and TLD results after repeat exposures
The results from the repeated experiment where fitted to a linear f(x) = ax + b
relationship using the least squares method. The results show a good agreement
with a linear relationship with an r2 value of 0.96. However the relationship is far
from the expected y = x with a slope a = 0.75 and an intercept b = 79. The results
from the OSLs at the higher exposure level (~1mSv) are higher than those of the
3 Results 49
TLD but agree well in the lower dose experiments. The high values for the OSL
doses cause the decrease in the slope of the fit in figure 3.10a and the high points
to the right of plot 3.10b. The results of other comparisons later in this chapter
support the conclusion that the reported OSL dose in this experiment were higher
than the actual value of a little under 1mSv for this exposure.
The spread of results between dosimeters increases with increasing dose. This
effect can be seen on all the subsequent plots. This could be explained by a systematic
bias on each individual dosimeter which becomes more evident with increasing dose.
The spread of results is greater for the OSL dosimeters at the highest dose exposure
but less than that of the TLDs at the lower dose levels. The spread of results
also appears to increase when turning the dosimeters from 0 to 30° and 60° from
perpendicular to the source.
The Bland-Altman plot including the results of the repeated high dose experiment
(figure 3.10b) shows the mean difference of -21μSv, and just two data points (of 51)
outside the 1.96σv of 106μSv . At the low and medium dose levels the OSL results
are below those of the TLD resulting in a negative mean difference. For the high
dose results however 5 of the 6 points are above the mean and 4 of the points are
positive.
The higher OSL results at the high dose level are obvious in the Bland-Altman
plot, as the mean difference is below zero but five of the six points from the high
exposure experiment are above the mean.
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3.2.6 Comparison of active dosimeters with OSL dosimeters
(a) Direct comparison of OSL and MGP results (b) Band-Altman plot
Figure 3.11: Comparison of OSL and MGP results
The agreement between the active dosimeters and the OSL dosimeters appears to
be better than that between the two types of passive dosimeters, with a = 1.18,
b = −49 and an r2 of 0.99. The OSL dosimeters report a higher dose than the
active dosimeters around 1mSv, excluding those data points would give a fit closer
to y = x.
The mean difference between active and passive dosimeters is just 3μSv with a
1.96σv of 125μSv. Six data points outside 1.96σv in a sample size of 84 is unexpected
and all occur at the highest dose level. The active and passive dosimeters show good
agreement in the low and medium dose exposures but an obvious difference at 1mSv
(over 200μSv difference in one case). The results from the OSLs are mostly lower
than the results from the active dosimeters at lower doses. At these lower doses
(<0.5mSv) there is some overlap in the reported doses from the two dosimeter types.
This can be seen in the points clustered around y=0 in figure 3.11b. The results
from the OSLs are all higher than the results from the active dosimeters in the high
dose experiment, this is obvious from the points on the right of figure 3.11b, which
are all below the line of y=0.
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3.2.7 Comparison of active dosimeters with Thermolumines-
cent dosimeters
(a) Direct comparison (b) Bland-Altman plot
Figure 3.12: Comparison of TLD and MGP results
The agreement between the active dosimeter and TLD results seems excellent, with
the slope of the linear fit to the data very close to unity, and an intercept near zero.
There are two sets of high dose comparisons because usable data comparing TLDs
and the active dosimeters was obtained in both the initial and repeated high dose
experiments.
The Bland-Altman plot also shows excellent agreement between TLDs and MGPs
in the controlled experiments. The mean difference is 0μSv, and the 1.96σv value is
105μSv. There is a spreading of the results as dose increases but only 3 points outside
the ±1.96σv values (one at ~350μSv and two at ~900μSv) which is to be expected for
a sample of 92 points. The agreement between the TDLs and the active dosimeters
is the best agreement between the three dosimeter types.
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3.2.8 Summary of inter and intra-dosimeter type compar-
isons in controlled experiments
Slope of Fit Intercept Coefficient of Determination (r2)
OSL-TLD 0.75 79 0.96MGP-OSL 1.18 -49 0.99MGP-TLD 0.99 1 0.97
Table 3.5: Summary of linear fits to comparisons of dosimeter results
On average the TLD and active dosimeters show the best agreement in a direct
comparison, with a slope of close to one, and an intercept close to zero. All of the
data sets show good agreement with a linear fit.
Mean difference 1.96 Standard Deviations Points of comparison(μSv) from the mean (μSv)
OSL -3 92 22TLD 9 109 21MGP -4 42 64
OSL-TLD -21 106 51MGP-OSL -3 125 84MGP-TLD 0 105 92
Table 3.6: Mean difference and 1.96σv values for dosimeter comparisons
Standard one sample t-tests show that of all these comparisons, the only sta-
tistically significant mean difference is the one between the two types of passive
dosimeter (OSL-TLD in table 3.6). The differences in mean dose results between
active and passive dosimeter results are not statistically significant for either passive
dosimeter type with a 95% confidence interval.
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(a) All Results (b) Results below 500μSv
Figure 3.13: Mean dose per Dosimeter Type vs Mean dose per exposure
The plots in figure 3.13 show the variation in each group of dosimeters and the
overall agreement of the dosimeters with each other. The figures plot the mean dose
reported by each dosimeter type, with bars showing the maximum and minimum
dose result reported, against the mean reported dose for all dosimeter types used
in an experiment. For the lowest doses reported the means agree exactly as they
are the same value, only the active dosimeters report any result at this level. In all
other cases there are clusters of vertically overlapping results as the maximum dose
for one dosimeter type overlaps the minimum dose for another. The closely grouped
vertical lines of results arise from the 0, 30 and 60 degree exposures at approximately
the same dose. The greatest differences between the groups of results arise at the
highest doses. The active dosimeter and TLD results for one high exposure overlap,
but the OSL results for that same exposure are very low, as reported earlier. The
low OSL results lower the overall mean, taking the TLD and active dosimeter results
above the line of x = y. The results for the repeated high dose experiment (at the
right of figure 3.13a) show the opposite result, with the OSL results higher than the
TLD and active dosimeter results, in this case though the TLD and active dosimeter
results do not agree as well, as the lowest active dosimeter result is higher than the
highest TLD result. In order to better display the lower dose results, figure 3.13b
shows the results below 500μSv.
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3.2.9 Statistical significance of agreement of means
T-tests were performed, as outlined in section 2.4.8, to test the significance of agree-
ment between the mean results of the different dosimeter types. Test were performed
for the low (~150μSv) and medium (~300μSv) dose experiments (experiments 4-6
and 7-9 in table 2.3). Dose results from the experiments with the dosimeters at 30°
and 60° were normalised using the method explained in section 2.4.6 and included
in the comparison in order to produce a useful number of results for each test.
Low Dose Low Dose Low Dose Med Dose Med Dose Med DoseMGP-OSL TLD-OSL MGP-TLD MGP-OSL TLD-OSL MGP-TLD
Difference20.6 12.1 8.53 4.57 26.3 21.8
of MeansStandard
9.53 12.0 8.96 9.33 18.7 18.5Error
Degrees of10.7 15.0 9.81 12.9 10.3 10.1
FreedomTest
2.17 1.01 0.95 0.49 1.41 1.18Statistic
Confidence(-0.33,41) (-13,38) (-11,29) (-16,25) (-15,68) (-63,20)
IntervalAgreement Yes Yes Yes Yes Yes Yes
Table 3.7: T-test Results (for p=0.05) for agreement of different dosimeter types
Table 3.7 shows no statistically significant difference in the mean results of the
two dosimeter types in each comparison, within the 95% confidence interval. The
confidence interval spans zero in all cases, but only just in the case of the low dose
comparison of active and OSL dosimeters.
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3.3 Comparison of staff doses recorded by passive
and active dosimeters
3.3.1 RAPID Staff doses
Staff in RAPID have been using the MGP active dosimeters together with the
logging stations since 2012. The following Bland-Altman plots show the level of
agreement between the active and passive dosimeters for 2012, 2013 and 2014. For
privacy reasons the staff are represented by numbers rather than by name. The use
of different symbols gives an indication of agreement between active and passive
dosimeters for different staff members. Each point is a comparison of the monthly
TLD result with the sum of the active dosimeter results over that month for the
same staff member.
In 2013 all staff using the MGP dosimeters were asked to wear the passive and
active dosimeters in the same location on the torso to minimise the differences
between the results. The survey results in table 3.17 show around 70% reported
compliance with this request at the time of the survey (August 2014).
3.3.1.1 2012
The first full year using the logging stations and database was 2012. One TLD result
has been excluded from these plots as it was found that the dosimeter was exposed
while it was not being worn, invalidating the result.
The r2 value of 0.49 indicates that the data dose not fit particularly well to
the least-squares linear fit. There are a large number of TLD results of “below
the detectable limit” (BDL) which is given by the manufacturer as 50μSv. These
results form the horizontal line in the bottom left of figure 3.14a extending out to
177μSv. The lowest reported TLD result is 100μSv. Excluding the comparisons
where the TLDs returned a value of “below the detectable limit” dose not improve
the correlation, r2 drops to 0.38 for the best fit shown with a dashed line. The slope
of the fit excluding the BDL results is closer to one but the intercept is much further
from zero.
In the Bland-Altman plot shown (figure 3.14b) the mean and standard deviation
values were calculated excluding the BDL results. As this excludes a large amount
of data (35 out of 117 points), calculations of the mean difference and the standard
deviation of the differences were calculated assuming three different values for the
unreported TLD results. A one sample t-test shows that the difference between the
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(a) Direct dose comparison (b) Bland-Altman Plot
(c) Bland-Altman Ratio Plot
Figure 3.14: Comparison of RAPID staff dose results for 2012
passive and active dosimeter results is statistically significant using a 95% confidence
interval.
As the stated minimum detection level is 50μSv, values of 0μSv, 25μSv and 50μSv
were used to cover the assumed range of potential values. The results of these
calculations are displayed in table 3.8.
Excluding the BDL results, the mean difference between results from the TLDs
and the summed active dosimeter results for each month was -190μSv with a 1.96σv
of 500μSv. The mean difference and spread of results is much larger than in the
controlled experiments. Picking a value between 0 and 50μSv for the unreported
3 Results 57
Excluding BDL BDL values BDL values BDL valuescomparisons set to 0μSv set to 25μSv set to 50μSv
Mean Difference -190 -116 -124 -1311.96 x Std Deviation 500 477 467 458
Table 3.8: Bland-Altman Results for 2012 RAPID Doses
TLD results reduces both the mean difference and the spread of differences. The
best agreement comes from setting the BDL results to 0μSv.
The Bland-Altman ratio plot (figure 3.14c) shows that on average the active
dosimeter result is 0.60 of the equivalent TLD result. The spread is wide with a
1.96σv value of 0.49. In only two of the comparisons is the active dosimeter result
greater than the passive dosimeter result. The points form a better horizontal fit
than the standard Bland-Altman plot, with what seems to be a consistent spread
with increasing average reported dose. This plot does not include any BDL values.
A one sample t-test shows that the ratio between the passive and active dosimeter
results is significantly different to 1 using a 95% confidence interval.
3.3.1.2 2013
The least-squares fit is closer to x = y than for the 2012 comparison but the
correlation coefficient suggests that the fit is no better. Again the active dosimeter
results report doses far in excess of 50μSv for a number of TLD reports of “below
the detectable limit” and the lowest reported TLD result is 100μSv. In the Bland-
Altman plot the mean difference is -136μSv and the 1.96σv is 321, an improvement
on the 2012 results. This may be due to the request to wear dosimeters in the same
location reducing the variation in exposure to the two dosimeters. A one sample
t-test shows that the difference between the passive and active dosimeter results is
statistically significant using a 95% confidence interval.
Excluding BDL BDL values BDL values BDL valuescomparisons set to 0μSv set to 25μSv set to 50μSv
Mean Difference -137 -100 -101 -1091.96 x Std Deviation 321 324 318 312
Table 3.9: Bland-Altman Results for 2013 RAPID Doses
As with the 2012 results, picking a value between 0 and 50μSv for the unreported
TLD results reduces both the mean difference and the spread of differences. The
effect on the spread of differences is not as great as in 2012 and again, the best
agreement comes from setting the BDL results to 0μSv (table 3.9).
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(a) Direct dose comparison (b) Bland-Altman Plot
(c) Bland-Altman Ratio Plot
Figure 3.15: Comparison of RAPID staff dose results for 2013
The Bland-Altman ratio plot (figure 3.15c) shows that on average the active
dosimeter result is 0.58 of the equivalent TLD result, this is similar to the 2012 result.
The spread is even wider with a 1.96σv value of 0.62. There are more instances of
the active dosimeter results being higher than the passive results, particularly some
results for staff members 7 and 4. Both staff members also have many comparisons
for the year where the TLD results are higher than the active dosimeter results,
illustrated by points below 1 on the y-axis. The spread of results seems to reduce
with increasing average reported dose, but the smaller number of points at higher
3 Results 59
doses make it difficult to draw conclusions. This plot does not include any BDL
values. A one sample t-test shows that the ratio between the passive and active
dosimeter results is significantly different to 1 using a 95% confidence interval.
3.3.1.3 2014
(a) Direct dose comparison (b) Bland-Altman Plot
(c) Bland-Altman Ratio Plot
Figure 3.16: Comparison of RAPID staff dose results for 2014
For the 2014 data the least-squares fit has a slope close to one and the correlation
coefficient is improved over 2012 and 2013. It can be seen in the plot that there are
a smaller number of points with very large differences between the TLD and active
3 Results 60
dosimetry results in the 2014 data, compared to previous years. The lowest reported
TLD dose is 120μSv.
Excluding BDL BDL values BDL values BDL valuescomparisons set to 0μSv set to 25μSv set to 50μSv
Mean Difference (μSv) -90 -50 -57 -631.96 x Std Deviation 169 185 175 167
Table 3.10: Bland-Altman Results for 2014 RAPID Doses
The spread of results has decreased year on year. In all three cases the results
from the active dosimeters are lower than those from the TLDs but are getting closer
each year. One sample t-tests show that the difference between the passive and
active dosimeter results is statistically significant using a 95% confidence interval for
all three years of results.
As with the previous results, picking a value between 0 and 50μSv for the unre-
ported TLD results reduces the mean difference, however for the 2014 results the
spread of differences is increased by setting the BDL values to zero or 25μSv. The
best agreement for the mean of the results again comes from setting the BDL results
to 0μSv (table 3.10).
The Bland-Altman ratio plot (figure 3.16c) shows that on average the active
dosimeter result is 0.56 of the equivalent TLD result, this is similar to the results
from the previous years. The spread is similar to 2012 with a 1.96σv value of 0.51.
The results do not seem distributed along the mean, showing that the ratio between
the active and passive results is not consistent. There is one very obvious outlier from
staff member 19 which is not easy to see in figures 3.16a and b as it is at a relatively
low dose level. This plot does not include any BDL values. A one sample t-test
shows that the ratio of the passive and active dosimeter results is again significantly
different to 1 using a 95% confidence interval.
3.3.1.4 The effect of reported wear position on correlation
It was thought that staff wearing the dosimeters in different locations on the torso
may be a contributing factor to the differences between the passive and active
dosimetry results. To assess this, comparisons (figure 3.17) were made of the results
only for the staff who, when surveyed, reported wearing both dosimeters in the same
location on their torso.
Excluding the staff who wear dosimeters in different locations does nothing to
improve the correlation. The linear fit has a similar slope and intercept to the
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(a) Direct dose comparison (b) Bland-Altman Plot
Figure 3.17: RAPID staff dose results for 2014 for staff wearing the passive andactive dosimeters in the same position on the body.
inclusive plot (figure 3.16) and the correlation to that line is worse than for the
inclusive plot. In the Bland-Altman plot the mean difference is increased from
-90μSv to -102μSv and the 1.96σv value is decreased from 169μSv to 159μSv by
including only the results from dosimeters worn in the same body position. A one
sample t-test on this data shows that the difference between the passive and active
dosimeter results is still statistically significant using a 95% confidence interval.
3.3.2 PET Centre Staff doses
3.3.2.1 2013
Staff in the PET Centre started using the dosimeters in 2013. As discussed in 2.5.2
the 2013 data was gathered with a large number of staff only wearing the active
dosimeters when working in PET. The staff were wearing their TLDs in PET and
also in Nuclear Medicine.
The TLD results are much higher than those from the active dosimeters. The
least-squares fit is meaningless with a correlation coefficient as low as 0.13 (figure
3.18a).
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(a) Direct dose comparison (b) Bland-Altman Plot
(c) Bland-Altman Ratio Plot
Figure 3.18: Comparison of PET Centre staff dose results for 2013
The Bland-Altman plot (figure 3.18b) demonstrates the poor correlation between
the TLD and active dosimeter results. There is only one active dosimeter result
less than the equivalent TLD result and the difference between the TLD and active
dosimeter results is marked and increases with increasing dose. The mean difference
between TLD and active dosimeter results is similar to that in the RAPID group
despite much lower average dose results. Setting the BDL values to 0, 25 or 50μSv
reduces the mean difference but the standard deviation is even larger (table 3.11).
The Bland-Altman ratio plot (figure 3.18c) shows that on average the active
dosimeter result is 0.36 of the equivalent TLD result, this is much lower than the
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Excluding BDL BDL values BDL values BDL valuescomparisons set to 0μSv set to 25μSv set to 50μSv
Mean Difference -125 -66 -75 -841.96 x Std Deviation 138 200 182 165
Table 3.11: Bland-Altman Results for 2013 PET Centre Doses
RAPID results. The spread is similar to RAPID with a 1.96σv value of 0.42. There
is no discernible trend in the data. This plot does not include any BDL values.
3.3.2.2 2014
As explained in section 2.5.2, in 2014 a smaller number of staff wore the active dosime-
ters in both PET and Nuclear Medicine in an attempt to gather more meaningful
data.
The direct comparison plot (figure 3.19a) does show an improved correlation
compared to 2013 but it is still far from convincing. The slope of the fit suggests
that the active dosimeter results are still much lower than those from the TLDs. In
the Bland-Altman plot (figure 3.19b) the mean difference is -86μSv and the 1.96σv is
93μSv. Given the lower doses recorded by the dosimeters compared to the RAPID
staff, one would expect that the difference and spread would be lower for PET centre
workers. The results excluding the “below detectable limit” results, are very similar
for the PET centre and RAPID in 2014, the mean difference and standard deviations
are both within a few micro-sieverts of the results from the other group.
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(a) Direct dose comparison (b) Bland-Altman Plot
(c) Bland-Altman Ratio Plot
Figure 3.19: Comparison of PET Centre staff dose results for 2014
Excluding BDL BDL values BDL values BDL valuescomparisons set to 0μSv set to 25μSv set to 50μSv
Mean Difference (μSv) -86 -52 -58 -651.96 x Std Deviation 93 140 123 108
Table 3.12: Bland-Altman Results for 2014 PET Centre Doses
For the 2014 results the mean difference is improved by assuming a value for the
BDL results and including the comparisons, but including the assumed BDL values
increases the standard deviation in all cases (table 3.12).
3 Results 65
The Bland-Altman ratio plot (figure 3.19c) shows that on average the active
dosimeter result is 0.56 of the equivalent TLD result, this is much more like the
RAPID results than the previous year’s results. The spread is similar to 2013 with
a 1.96σv value of 0.43. The two points where the active dosimeter reading was higher
than the TLD results have increased the mean ratio. Without those two points the
mean is reduced to 0.52. This plot does not include any BDL values.
One sample t-tests show that the difference between the passive and active
dosimeter results are statistically significant using a 95% confidence interval for both
the difference and ratio data for 2013 and 2014.
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3.4 User Survey Results
A copy of the user survey can be found in Appendix A. 22 questionnaires were
completed.
3.4.1 Profession
Profession Number
Technologist 9Nurse 5
Radiochemist 7Other (Engineer) 1
Table 3.13: Professions of those surveyed
The technologists and nurses work in the PET center. The radiochemists and engi-
neers work in RAPID. The radiochemists include those that do regular productions
of 18FDG and those who produce research radiopharmaceuticals.
3.4.2 Time using Passive and Active Dosimeters
Timespan Passive Active
Less than 3 months 33-6 months6-12 months 1 1
Longer than 12 months 21 18
Table 3.14: Experience using Active and Passive Dosimeters
Almost all of the workers have been wearing both types of dosimeter for more than
12 months and so are accustomed to wearing them.
3.4.3 Ease of use of Dosimeters
Ease of Use Passive Active
Very Easy 18 174 30 20 0
Complex/Difficult 0 0
Table 3.15: Ease of Use of Dosimeters
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Nobody reported finding the dosimeters difficult to use.
3.4.4 Comfort wearing Dosimeters
Passive Active
Very Comfortable 14 135 82 11 0
Difficult/Uncomfortable 0 0
Table 3.16: How comfortable are dosimeters
Almost all the staff reported finding both dosimeters comfortable to wear.
3.4.5 Wear Position of Dosimeters
Passive Active
Waist 13 9Chest 9 13
Same for both 16
Table 3.17: Wear Position of Dosimeters
There is a preference for wearing the active dosimeters at chest height and the
passive dosimeters at the waist when staff wear them in different positions. 16 of 22
staff members wear the dosimeters in the same location (as requested), evenly split
between waist and chest.
3.4.6 Frequency of checking results
How often Passive
Every Month 11Most Months 5Sometimes 4
Hardly Ever 2Never 0
How often Active
Every Day 18Most Days 2Sometimes 1
Hardly Ever 0Never 1
Table 3.18: Frequency of checking dosimeter results
Results from the passive dosimeters are only available once per month and half
the staff report checking the results every month. Most staff report checking their
3 Results 68
results from the active dosimeter every day, probably because it is displayed on
screen during log out.
3.4.7 Level of trust in dosimeter results
Level of trust Passive Active
Completely 2 611 126 32 0
Not at all 0 0Did not respond 1 1
Table 3.19: Level of trust in dosimeter results
The overall level of trust in the results is high with slightly more trust in the results
of the active dosimeter.
3.4.8 Rate of not wearing a dosimeter
Passive Active
Never 7 6Hardly Ever 9 11Sometimes 2 3
Once per month 3 0Once per week 1 1
N/A 0 1
Table 3.20: The rate at which workers forget to wear dosimeters
Workers report a high level of compliance with wearing dosimeters. The N/A result
is from a nurse in the PET Centre who said that she had stopped wearing an active
dosimeter.
3 Results 69
3.4.9 Usefulness of results and feedback
Passive Active
Very Useful 4 176 26 24 1
No use at all 2 0
Table 3.21: Usefulness of results and feedback
The feedback from the active dosimeters is clearly seen to be more useful than that
from the passive dosimeters.
3.4.10 Prefer to wear Active, Passive or Both
Number
Just Passive 1Just Active 14
Both 7
Table 3.22: Prefer to wear active, passive or both
Given the choice most of the staff questioned would prefer to use just an active
dosimeter, about one third would like to use both and only one member of staff
would prefer to use just a passive dosimeter.
3.4.11 Additional Comments
The following comments were added by the staff:
Finger badges cannot be replaced by electronic dosimeter
TLD result turn-around too slow.
Log-in problems made use of MGPs more difficult
MGP recommended for high radiation areas
National dosimetry register to collect data from all providers using com-
mon unique identifier for workers.
Audible alarm helpful reminder, good for students, possible to equalize
dose among staff by assigning high dose procedures to staff with lowest
dose.
3 Results 70
Don’t wear MGP as end of day alert is annoying
TLD is retroactive so too late. MGP better as checking each day makes
you more aware.
Like the beep as it is a constant reminder. If switch to just electronic
would still want monthly summaries.
Not had any really helpful info back from the MGPs. Like the beeps as
it is a constant reminder.
The comments mainly center on the audible feedback from the active dosimeters,
the delay in reporting from passive dosimeters and some technical issues with the
active dosimeters. The reference to finger badges acknowledges that current active
monitoring systems cannot replace passive dosimeters entirely, as there are no sys-
tems to record extremity doses. RAPID workers are required to wear ring badges
containing TLDs to monitor their extremity doses.
Chapter 4
Discussion
4.1 Reliability
In the course of the controlled experiments there were occasions where passive
monitors produced unexpectedly low results. As shown in table 3.2 a number of
results were excluded from comparison because the reported dose was given as “below
detectable limits” when the results from the other two detection methods were above
the stated detection limit of the passive monitor. In one case a single TLD badge
from a batch reported “below detectable limits” while the other badges of the same
type reported doses around 120μSv, well above the detection limit.
One of the experiments had to be repeated as a set of results from the OSL
monitors came back as approximately one third the reported doses of the active and
TLD monitors. In the repeated high dose experiment the results from the OSLs
were higher than the TLDs and active dosimeters, but the difference was not nearly
as great as in the original experiment.
There were also incidents of staff doses measured by TLDs being unexpectedly
reported as “below detectable limits” when their work patterns were unchanged and
active dosimeters reported typical doses. This may reduce worker confidence in the
results they are receiving. Despite these problems the result of the staff survey in
table 3.19 show a high level of trust in the results though slightly lower for the
passive dosimeters than the active ones.
Given the relatively small number of passive dosimeters used, the number which
seem to have failed to report doses accurately is a cause for concern. The details are
summarised in table 3.2. This lack of reliability does not seem to have been reported
71
4 Discussion 72
in the literature and is worthy of further investigation. Regular blind testing of
dosimetry services using controlled exposures is recommended (Böhm et al., 1994)
but there is no evidence in the literature that this has been carried out in Australia
in recent history.
All of the active dosimeters performed reliably during the controlled experiments.
There were a few older active monitors that failed (while in use in the PET centre)
during the course of this investigation, but their failure was immediately obvious
to users, and so did not significantly reduce the effectiveness of dose monitoring.
If there are spare batteries and a few spare monitors for when there are technical
problems, the reliability of the active dosimeters seems higher than that of the
passive dosimeters in terms of service provision. The database and logging stations
can pose a single point of failure, but with backups a problem of this nature should
pose no greater disruption to monitoring than a shipment of passive dosimeters being
delayed in the mail.
4.2 Equivalence
The results of the controlled experiments show excellent agreement between the
active and passive dosimeters over a range of exposures typically seen by radiation
workers. The data shows no more variation between active and passive dosimeters
than there is difference between the two types of approved passive dosimeter either
perpendicular to the radiation source or at 30 and 60 degrees to the source. The
only statistically significant difference in mean dose comparisons was between the
two types of passive dosimeter (section 3.2.8). The high level of agreement between
the TLD and active dosimeter results (figures 3.12a and b) suggest that differences
between the active and OSL dosimeter results are likely to be because of the OSL
dosimeters over-reporting the dose at around 1mSv. Without the high dose results
the spread of results in the Bland-Altman plot (figure 3.11b) would be much smaller.
When assessing the quality of a dosimetry service the acceptable difference be-
tween the expected result and that reported can be ±50% (Böhm et al., 1994) for
the kinds of relatively low doses experienced by radiation workers. The difference
between the results from active and passive dosimeters in the controlled experiments
was always less than 50% and usually much less than that. The spread of results
from the passive dosimeters as seen in figures 3.2.2a and b was very similar to the
spread between active and passive dosimeters as seen in figures 3.11 and 3.12. The
spread was also similar when comparing the two types of passive dosimeter (figure
4 Discussion 73
3.10). The results are summarised in table 3.6. It is clear from these results that
in controlled conditions with 511keV gammas there is no greater difference between
the active and passive dosimeters than there is between the two types of approved
passive dosimeters or even between different individual passive dosimeters of the
same type. The active dosimeters are at least as accurate at reporting dose as the
passive dosimeters in these conditions.
A premises using radioactive material in WA must have at least one calibrated
dosimeter for measuring dose rates (Western Australia, 1984). When calibrating
dosimeters, settings on the dosimeter must be adjusted to attempt to bring the
measured dose rate within 10% of the actual dose rate. If no adjustments are
capable of bringing the measured dose rate within 20% of the actual dose rate the
dosimeter fails and cannot be used. Calibration certificates are provided specifying
the isotope or x-ray energies used to calibrate the dosimeter and its results compared
to the reference dose rates. In the controlled experiments we do not have a reference
dose to compare the results to, as no laboratory calibrates dosimeters to 18FDG. The
best comparison for evaluating the suitability of the active dosimeters appears to be
the comparison with TLDs, due to the large number of OSLs which failed to provide
usable results, and the apparent over-reporting of the ~1mSv results. Even in the
best comparison (figure 3.12) some of the results of the TLDs and MGPs differed
by more than 20% of the average of their results. Much of the difference between
the results are due to the spread of results from the passive dosimeters. The spread
of results from each dosimeter type is shown in figure 3.2.2, the active dosimeters
show an obvious advantage in terms of precision.
The comparison of active and passive dosimeter results from workers is far less
conclusive than the controlled experiments. The results from the active dosimeters
were almost always lower than those from the passive dosimeters except when the
passive dosimeters reported “below detectable limit”. This bias could be a result of
failure to always wear an active dosimeter when exposed. This was definitely the
case for the 2013 PET centre results as the active dosimeters were only worn in PET
and not in Nuclear Medicine. It is also possible that the large differences in the
results are due to large anisotropies in the radiation field caused by uneven shielding
and differing positions of the two badges when working. As shown in table 3.17,
not all staff wore the two badges in the same position. When working with small,
high activity sources even small distances can make large differences to exposure of
different areas. This explanation is undermined by comparing the results shown in
figures 3.16 and 3.17. Limiting the comparison to the staff who reported wearing
4 Discussion 74
dosimeters in the same location on the body did not improve the correlation between
the results of the active and passive dosimeters.
It should be noted that there are significant errors in attempting to measure an
individual’s exposure using a personal radiation monitor of any kind. In laboratory
conditions it is usually possible to measure radiation fields to within 10% of the true
value (International Commission on Radiological Protection, 1997). In the workplace
non-uniformity and uncertain orientation can change the recorded dose by a factor of
1.5 in either direction (International Commission on Radiological Protection, 1997).
In light of this, it seems that equivalence should be judged more by the experiments
in controlled conditions than the workplace comparisons.
4.3 Repeatability
When measuring any quantity it is important that the measuring instrument reports
the same result every time it measures the same quantity. The repeatability of
each type of dosimeter can be judged by the spread of results when measuring the
same radiation dose. The Bland-Altman plots in figure 3.1 show that the average
difference between results from the same type of dosimeter are smallest for the active
dosimeters. The 2σv value for the MGPs was approximately half that of the 2σv value
for the OSL and TLD results.
4.4 Limits of Detection
The purpose of personal radiation monitoring is to provide information on the dose
individual staff members are occupationally exposed to. Radiation regulations and
standards are based around the principle of keeping radiation exposure As Low As
Reasonably Achievable (the ALARA principle) (International Commission on Radi-
ological Protection, 1997). Dose reports can be examined to highlight unexpectedly
high doses in radiation workers. The data from the staff monitoring in section 3.3.1
contained no reported staff doses from TLDs below 100μSv but a large number of
reports of “below the detectable limit”. This would suggest that the true lower
limit of detection for the TLDs used in the workplace is around 100μSv. From one
monitoring period to the next an individual’s exposure could vary from 0 to 100μSv
without any indication that changes in their work practice was exposing them to
more radiation. This is clearly not the case for an active dosimetry system that
displays reading down to 1μSv at the end of each day. The primary justification for
4 Discussion 75
a dose monitoring system is in the way in which it helps to achieve and demonstrate
an appropriate level of protection (International Commission on Radiological Protec-
tion, 1997). This present study would suggest that the use of an active monitoring
system could allow better compliance with the ALARA principle.
4.5 User compliance
Even a technically perfect dosimeter can only report the dose to the worker if it
is worn during any work involving sources of ionising radiation. There is evidence
to suggest that large numbers of radiation workers in healthcare regularly fail to
wear their dosimeters (Klein et al., 2015; Padovani et al., 2011). Compliance with
the requirement to wear a passive dosimeter is difficult to enforce, relying on spot
checks and comparison of reported doses with those of similarly employed colleagues.
Many hospital radiation workers regularly report doses below the detectable limit of
passive dosimeters which means that there is no way to tell if a dosimeter has been
worn at all. Where an active dosimeter is coupled to a logging system however, it is
a simple matter of querying the database to check whether an employee has logged
in a dosimeter regularly, such records can then be compared to shift rosters or other
work records to monitor compliance. The user survey (table 3.20) suggests that
non-compliance among those working with PET radiopharmaceuticals and patients
is low, though self reporting may not be 100% reliable. Since the conclusion of the
data gathering phase of this thesis, work has been undertaken to systematically track
the logging in and out of staff in RAPID. This system allows us to better calculate
staff dose per work day and improve radiation hygiene, it also serves to highlight
non-compliance.
4.6 User Acceptance of Active Dosimeters
Active dosimeters are larger and heavier than passive dosimeters, they also require
more effort to use as they must be logged in and out every shift. Despite these
drawbacks they were accepted well by staff and preferred over passive dosimeters by
many, as demonstrated by the results of the user survey (section 3.22).
One staff member in the PET center stopped wearing the active dosimeter (she
was not obliged to wear one) as she found an alarm emitted after eight hours logged
in to be annoying (her shifts regularly exceeded eight hours). This problem could
have been overcome with changes to the settings of the dosimeter.
4 Discussion 76
Many workers responded that they found the audible feedback from the active
dosimeters to be useful and felt that it helped them to reduce their radiation exposure.
No evidence could be found to demonstrate a reduction in radiation dose after the
introduction of the active dosimeters. Any change could easily be lost in the noise
due to the variability in worker routine, dose results, patient numbers and dispensed
activity from month to month.
4.7 Approval of Personal Radiation Dosimetry Ser-
vices
The standards required of providers of personal dosimetry services vary around the
world (Cavallini et al., 1994; Marshal, 1998). The United States has had a program
of assessing service providers since the 1960s (Schauer et al., 2004), the current
testing requirements of the National Voluntary Laboratory Accreditation Program
(NVLAP) are given in HPS N13.11-2001 (Soares, 2007). In the United Kingdom the
Health and Safety Executive (HSE) also publish a set of requirements which must
be met by service providers (Health and Safety Executive, 2010). In jurisdictions
which mandate testing of service providers, batches of dosimeters exposed to known
doses are sent for analysis by the service providers. The results are examined for
compliance with a number of criteria. The US and UK both use pass/fail criteria
based on the bias in the average of the results and the standard deviation for batches
of dosimeters. The criteria used in the UK for dosimeters designed to monitor whole
body gamma exposure are shown in table 4.1. The service provider is required to
pass the initial assessment before being approved and are subject to repeat testing
every 5-7 years (Health and Safety Executive, 2010). A 2001 update to the NVLAP
performance criteria requires that no more than 10% of the dosimeters tested fall
outside the acceptance criteria for the mean of all results (Schauer et al., 2004).
This change prevents approval of services with a rate of dosimeter failure close to or
higher than 10%.
The Radiological Council of Western Australia do not mandate regular testing of
approved suppliers; service providers must request initial approval from the Council.
In the preparation of this thesis Council officers were asked about the methods used to
approve providers. The providers are asked to produce details of their measurement
methods and processes and demonstrate that measurement methods are traceable to
Australian or international standards. One of the most significant factors in gaining
approval is the provider having a national or international accreditation from one or
4 Discussion 77
Limit on Criteria (must pass all)
Bias in average of all results < 20%Standard Deviation in all results <10%
Bias in average of < 20%each group of 5 dosimeters (< 30% for any group irradiated to 1.0 mSv or less)
Standard Deviation of < 10%each group of 5 dosimeters (< 15% for any group irradiated to 1.0 mSv or less)
Table 4.1: UK HSE Pass/Fail criteria for dosimetry services for monitoring wholebody gamma exposure (Health and Safety Executive, 2010)
more of the major accreditation providers such as National Association of Testing
Authorities (NATA). NATA accreditation for personal dosimetry is in part based
on the provider meeting the standards set by the HSE, NVLAP or International
Accreditation New Zealand (IANZ).
In our controlled experiments we have no true dose values to compare our results
to so we cannot say if any of our sets of dosimeters would have passed a review to the
standards of the HSE or NVLAP. We can say however that the MGP results showed a
much smaller standard deviation that either of the passive dosimeter types. It would
be prohibitively expensive for an individual hospital to obtain NATA accreditation
as a dosimetry service provider. This may represent a significant hurdle to getting
the active dosimetry system approved by the Radiological Council as a means of
obtaining the legal dose record.
4.8 Standards for Personal Radiation Monitors
National standards relating to radiation protection and detection tend to be based
largely on international standards published by bodies such as the International
Atomic Energy Authority (IAEA), the International Commission on Radiation Pro-
tection (ICRP) and the International Electrotechnical Commission (IEC) (Voytchev
et al., 2011). The IEC has a sub-committee (45B) focused on radiation protection
instrumentation which has published standards for both active personal dosimeters
(IEC 61526 also known as ISO 4037) and passive integrating dosimetry systems (IEC
62387-1). The two IEC standards contain a list of criteria for the dosimetry systems
including the dosimeter reading and reporting equipment. The criteria include the
minimum energy range, and minimum and maximum dose and dose rate levels, over
which a dosimetry system must function. Type testing is not included in IEC 62387-
1; it is only concerned with system properties and states that absolute calibration
4 Discussion 78
should be performed as part of routine testing (Voytchev et al., 2011). Comparing the
requirements between these two standards show that the requirements for the passive
dosimeters are more stringent, with a broader energy requirement; 12 keV–7 MeV for
passive dosimeters and 80 keV–1.5 MeV for active dosimeters when measuring Hp10
for gamma exposure. This is not to say that active dosimeters cannot meet the same
standard as the passive dosimeters, but that they do not have to in order to satisfy
IEC 61526 Edition 3. Boziari and Hourdakis (2007) and others have demonstrated
that some active personal dosimeters show good energy independent response down
to 50keV, while with others the response drops sharply below 65keV making them
unsuitable for some applications.
The results of tests of a number of active personal dosimeters against aspects of
IEC 61526 have been published. Texier et al. (2001) tested the energy response of
seven different active dosimeters including the DMC 2000S, one of the dosimeters
used in this study. It was found that the majority conform to the standard but many,
including the DMC 2000S, have a poor response below 50 keV. Using the x-ray and
gamma energies suggested in IEC 61526, angular response was tested for two types
of active dosimeter by Suliman et al. (2010) and very little variation was found with
rotation which is in agreement with the results in section 2.4.6.
4.9 Calibration of APDs
Unlike passive dosimeters, active dosimeters may be in use for more than 10 years
(see section 4.12.3). There is potential for their performance to degrade over time,
or be altered by calibration or changes to settings. If the active dosimetry system
were to provide the legal record of occupational dose this problem would need to be
overcome through regular calibration. This could either be performed in-house, if a
suitable source and measurement apparatus is available, or by an approved service
provider against a source traceable to a national standard. This requirement for
calibration would place an extra workload and/or expense on the center using APDs.
4.10 Record Keeping and Data Analysis
Having a single database containing all the dose data for your staff has advantages
and disadvantages. Whatever dosimetry system is used as the legal record of worker
dose there is a legal requirement for data retention; in Western Australia personal
dosimetry records can not be disposed of without the explicit permission of the
4 Discussion 79
Radiological Council (Western Australia, 1984). A single database provides a single
point of failure if the database is not backed-up, posing a great risk of loss of data.
This is counteracted by the ease with which electronic files can be backed-up and
stored, in comparison to paper records. The main advantage of a single structured
database is the ease with which data can be queried and cross-referenced. The
reports provided by external suppliers have moved from paper to electronic files.
The files provided are of changing file format and structure which makes compiling
data from multiple years difficult, error prone and time consuming. Data analysis
is much easier with a single, large structured database than a large collection of
separate files.
4.11 Incident investigation
One of the significant advantages of active over passive dosimeters is in the area of
incident investigations. With passive dosimeters dose reports much be checked for
doses above regulatory limits or internal trigger levels and investigations of unusually
high reported doses must be carried out. Any results, either monthly or quarterly,
which are above the pro-rata annual limit (20mSv/year whole body dose) must
be reported to the Radiological Council in Western Australia (Western Australia,
1984). The delays in data collection and reporting with passive dosimeters, and the
inability to pinpoint the day a high dose occurred, can cause significant problems in
investigating and reporting on high doses.
Active dosimetry systems on the other hand can provide immediate notification
of unusually high exposure or dose rates. This acts to both to reduce exposure and
pin-point where and when the exposure occurred. If a high monthly dose is shown
without a record of a high exposure incident there will be a record of chronic, above
average exposure. Whether the exposure is acute or chronic, the dose record can be
a significant help when investigating and comparing work practices and technique
with those of other staff. A day by day record of received dose and maximum dose
rate can be a significant help in attempting to reduce radiation dose to workers.
4.12 Economic Comparison
Active dosimeters are required by the state regulator in Western Australia for cy-
clotron production of radiopharmaceuticals. This means that both active and passive
dosimeters are required by legislation. There is an obvious economic advantage to
4 Discussion 80
removing the requirement for passive dosimeters if active dosimeters can be demon-
strated to provide adequate protection alone. In other work areas where there is no
requirement for active monitoring, whatever the technical advantages of an active
dosimetry system it is unlikely to be adopted if it is prohibitively expensive when
compared to passive dosimetry.
This section is not intended as a full economic assessment of the available dosime-
try systems, but is intended to give an overview and impression of the costs involved.
As with any costings projecting years into the future, a number of assumptions about
future prices and equipment reliability must be made, based on past information.
A comparison of the costs of passive and active dosimetry systems is a comparison
of a small but ongoing cost, against a larger upfront cost, with the outlay going
forward being a requirement for equipment replacement. Any dose monitoring system
will also require staff time to manage, which can be non-trivial.
4.12.1 Costs of Passive Dosimetry
For each wear period, either one or three months, a dosimeter must be supplied and
returned for each member of staff. There is a delay between ordering and receiving
dosimeters, so usually a large department will have a standing order for a number
of spare dosimeters, to be assigned to new members of staff or to cover for lost or
damaged dosimeters. There is a fixed cost for each dosimeter supplied (excluding
control dosimeters). If any badge is not returned within a designated time, for
example within 1 month of the end of the wear period, it is considered lost and an
additional lost badge charge is levied. Some suppliers also charge a delivery fee, a flat
fee independent of the number of badges delivered. The cost per year of the passive
dosimetry system shown in table 4.2c are simply 12 times the unit cost, multiplied
by the number of staff and thus represent the minimum cost with no lost badges
or delivery fee. The dollar values given in the costings in table 4.2a are indicative
values based on 2015 figures quoted to staff in Medical Technology & Physics by
three of the main service providers in Australia. There is no significant difference
between the cost of TLD and OSL badges.
There is a great deal of time spent by staff receiving, checking, distributing,
collecting, packaging and returning dosimeters each wear period when there are
large numbers of dosimeters. Time is also spent making changes to orders due to
staff changes and processing the invoices to pay for each shipment. For a large
department with regular staff changes, managing communication with the supplier
can become a significant time burden and expense.
4 Discussion 81
4.12.2 Costs of an Active Dosimetry System
An active dosimetry system requires a number of components as explained in 2.3.
While the costs of a passive system scale roughly linearly with the number of workers;
due to the requirement for one or more logging stations and a central database, this
is not the case for an active system. Due to relatively large capital outlay, any
calculation of cost per worker per year is strongly dependant on the number of
workers and the lifetime of the various pieces of equipment. The costs per year in
table 4.2c are calculated by summing the the unit costs of the required equipment
divided by its predicted lifetime.
As with the passive dosimetry service there will be a need for monitoring staff
doses and following up on results. Far less time is spent in the occasional battery
change and replacement of faulty dosimeters than is spent in the routine handling of
large numbers of passive dosimeters. There would be the additional staff overhead
of report production, but this is a largely automated process.
4.12.3 Lifetime of MGP Active dosimeters
One of the important economic considerations is the lifetime of the dosimeters. The
date of purchase and failure of every MGP dosimeter used at Sir Charles Gairdner
Hospital was obtained to assess the typical lifetime of an active dosimeter of this
type.
4 Discussion 82
Figure 4.1: Lifetime of MGP dosimeters
It can be seen in figure 4.1 that the typical lifetime of the MGPs which have
failed is around 10 years. There are however three dosimeters still in use after 11
years and two more still active after 13 years. For this simple analysis a value of
10 years was chosen. The logging stations and PC used to run the database were
replaced in 2016 so a lifetime of 4 years was used.
4.12.4 Comparison of costs per year
With any set of reasonable assumptions it is clear from table 4.2c that for large
groups (> 20) an active dosimetry system can cost less per year than a passive
system. The difference in costs will however be outweighed easily by any change
in staff time allocated to managing the dosimetry system, analysing results and
performing investigations. As noted in section 4.11 investigation of high exposure
should be simpler and more effective with an active dosimetry system reducing the
duration of investigations and improving their effectiveness. Alternative dosimetry
systems may prove more cost effective in the future. Instadose dosimeters are a
4 Discussion 83
Unit cost (AU$)
Passive dosimeter 15MGP active dosimeter 800
Logging Station 3000Database PC 2000
Software 500
(a) Indicative unit costs for active and passive dosime-try systems in 2015 AUD
Lifetime (Years)
MGP active dosimeter 10Logging Station 4
Database PC 4
(b) Expected Lifetime of active dosimetry system com-ponents
Total staff requiring monitoring 5 12 12 25Staff requiring monitoring simultaneously 5 12 10 20
Cost (AU$/year)
Passive Dosimetry 900 2160 2160 4500Active Dosimetry 1775 2335 2175 2975
(c) Averaged cost per year of active and passive dosimetry systems for different usergroup sizes. Costs exclude staff costs in managing the system.
Table 4.2: Economic Comparison of Active and Passive Dosimetry
4 Discussion 84
small electronic personal dosimeter without a screen which can report doses to a
central database via the web when plugged into a PC. They are currently cost
competitive with passive dosimeters and their cost could fall with greater uptake of
the technology.
4 Discussion 85
4.13 Legislative issues in Western Australia
Every state has its own legislation regarding the use of ionising radiation and the
protection of personnel working with it. In Western Australia the relevant legislation
is The Radiation Safety Act 1975 and the Radiation Safety (General) Regulations
1983. The Act and Regulations are administered by a statutory body called the
Radiological Council (Western Australia, 1975).
The Act and Regulations were written before the advent of active personal dosime-
ters and because of this the wording of the Regulations relating to the requirements
for personal dosimetry could pose an impediment to the replacement of passive
dosimeters with active ones even if active dosimeters are deemed technically suitable
or even superior. There is nothing in the Regulations that specifically prohibits the
use of active dosimeters, the means by which dose information is obtained is not
described at all.
Section 25 of the Regulations relate specifically to personal monitoring devices,
in particular this section refers to use of “an approved personal monitoring device”
and the use of “the services of radiation monitoring organizations that have been
approved”. A list of approved organizations can be obtained from the Radiolog-
ical Council website, all of the approved organizations provide passive monitors
(Radiological Council of WA, 2010).
It is possible to request approval from the Radiological Council for a new ser-
vice provider but the approval of active monitors would be a significant shift from
current practice. At present an approved supplier supplies the badges and is re-
sponsible for reading them and reporting the doses. The approved suppliers are
either large laboratory organizations who have obtained internationally recognised
quality accreditations or their agents. With the active dosimetry system discussed
in this thesis the organization producing the reports is the organization employing
the workers, this could be seen by the Council as a conflict of interest. If the Council
were amenable to approving individual workplaces as approved personal radiation
monitoring service providers it would mean that each workplace would have to apply
individually. Applying for approval could prove a significant administrative bur-
den which could discourage centres from switching despite the advantages of active
dosimeters. Having to assess a large number of applicants for approval would also
place additional work on the Council and its Officers.
One type of electronic dosimeter, the Instadose (www.instadose.com), has been
approved by the Radiological Council. This dosimeter differs significantly from the
active dosimeters discussed in this thesis in that it offers no direct feedback. Its main
4 Discussion 86
advantage is that it can be read at any time by plugging it into a PC with the correct
software installed; the dosimeter does not need to be returned to the supplier. The
dose data is stored by the supplier on remote servers which receive updates from the
software. Account managers can access and assess the doses recorded through the
software. The use of such a system may overcome some of the issues with passive
systems, but it does not offer the full range of advantages of an active dosimetry
system and would still require RAPID workers to wear two dosimeters.
Chapter 5
Conclusion and Future Work
5.1 Conclusions
There seems to be no technical reasons why the active dosimetry system described in
this work could not be used to provide the occupational dose record for staff working
with PET radiopharmaceuticals and patients. Controlled experiments demonstrated
that the ability of the active dosimeters to measure radiation dose from 18FDG was at
least as good as that of the passive dosimeters currently in use at Sir Charles Gairdner
Hospital. The agreement between the active dosimeters in the same experiments
was better than that between the same type of passive dosimeters.
The controlled experiments demonstrated a problem with the reliability of the
passive dosimeters, with many results being significantly lower than expected. In
a real life situation this could lead to under reporting of occupational doses. Both
the controlled experiments and the staff dose results showed that the actual lower
limit on detection for the passive dosimeters is greater than that stated by the
suppliers. Over a year the unreported doses from passive dosimeters could total
1mSv and opportunities for dose reduction could be missed. If the project were to
be repeated it would be sensible to use a larger number of passive dosimeters to
gather better statistics and attempt to gather data for all dosimeter types over the
dose range of interest. When designing the experiments such a large failure rate was
not anticipated, in particular the failure of multiple dosimeters in the same batch.
The nature of gathering workplace data meant that few conclusions could be
drawn from comparing active and passive dose results for staff. The correlation
between the dose results for active and passive dosimeters was far from clear, though
it did seem to improve over the course of the monitoring. A large proportion of
the results were below the detection limit of the passive dosimeters and so gave no
87
5 Conclusion and Future Work 88
meaningful data. There was an obvious overall lower dose recorded by the active
dosimeters. Gathering more informative workplace data would require a change to
work practices such that workers wore the active dosimeters whenever they were at
work and more supervision to ensure that there were not periods where they were
not being used. The problems with doses below 100μSv not being recorded by the
passive dosimeters are always going to result in a smaller pool of useful comparisons.
With one exception, staff reported that they were comfortable using the active
dosimeters and many stated a preference for the active over passive dosimeters.
For a medium to large workforce (20 or more radiation workers) there should be
no economic disincentive to switching to an active dosimetry system if a long term
(multi-year) view is taken.
The major hurdle is legal approval. Active dosimetry systems have been approved
in some jurisdictions but the only system approved for use in Western Australia uses
dosimeters which simply store data and report it back to a central service without
relying on local data storage and report production. No currently approved personal
dosimetry systems give instant feedback of dose or dose rate, which is one of the
major advantages of active personal dosimetry.
5.2 Future Work
The number of passive dosimeters which reported doses well below those expected in
the controlled experiments suggests that further work be carried out to determine how
significant this problem is. A wider survey involving larger numbers of dosimeters,
exposed to a range of known doses, should be carried out. Such a survey is probably
the remit of state and national regulatory bodies. A program of regular assessment
similar to the US model (Böhm et al., 1994) may be required to assure that personal
dosimetry services are providing the expected results.
The next obvious step for this work is to apply to the Radiological Council
for approval of the active personal dosimetry system. If approval can be obtained
for use in the RAPID and/or Nuclear Medicine areas then attention could turn
to other areas in the hospital who might wish to use an active dosimetry system.
Radiation Oncology, Radiology and Cardiovascular Medicine all have large numbers
of radiation workers. Each area has the potential for significant exposure either
through routine work or accident situations and may benefit from the feedback from
active dosimetry.
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Appendix A
User Experience Survey
94
If you have any questions please contact Steve on x1450 or [email protected]
Personal Radiation Monitoring Questionnaire
This questionnaire is intended to gather data as part of a project to determine whether
electronic personal dosimeters can replace passive dosimeters (TLDs) for staff working
with PET radiopharmaceuticals. The data you provide will help determine whether
SCGH will pursue the replacement of TLDs with electronic dosimeters. The introduction
of electronic dosimetry will only happen if it is technically appropriate, satisfies legal
requirements and is desirable to the staff of SCGH.
Your Name …………………………………………………………………..
Your name will not be included in any publications; it is collected for data analysis only.
Are you a…
Technologist [ ] Nurse[ ] Physician[ ] Radiochemist[ ] Other[ ]
How long have you been using passive dosimeters (TLDs, OSLs, film badges)?
Less than 3 months [ ] 3-6 months [ ] 6-12 months [ ] longer than 12 months [ ]
How long have you been using the electronic dosimeters?
Less than 3 months [ ] 3-6 months [ ] 6-12 months [ ] longer than 12 months [ ]
How would you rate the ease of use of TLD badges and MGP Electronic dosimeters?
Very Easy Complex/Difficult
TLD 1 2 3 4 5
Electronic 1 2 3 4 5
How comfortable is it to wear the dosimeters?
Very Comfortable Difficult/Uncomfortable
TLD 1 2 3 4 5
Electronic 1 2 3 4 5
Where do you normally wear the dosimeters? (mark with an X)
TLD Electronic
Please continue overleaf
If you have any questions please contact Steve on x1450 or [email protected]
How often do you check your results?
TLD Every month [ ] Most months [ ] Sometimes [ ] Hardly ever [ ] Never [ ]
Electronic Every day [ ] Most days [ ] Sometimes [ ] Hardly ever [ ] Never [ ]
How much do you trust the results?
Completely Not at all
TLD 1 2 3 4 5
Electronic 1 2 3 4 5
Roughly how often do you forget to use a dosimeter?
TLD Never [ ] Hardly ever [ ] Sometimes [ ] Once per month [ ] Once per week [ ]
Electronic Never [ ] Hardly ever [ ] Sometimes [ ] Once per month [ ] Once per week [ ]
How useful are the results or feedback from the dosimeters in monitoring and reducing
your exposure? Very Useful No use at all
TLD 1 2 3 4 5
Electronic 1 2 3 4 5
Given the choice would you rather…
use just the TLD [ ] use just the Electronic Dosimeter [ ] use both [ ]
Please add any other comments…
Appendix B
Example Dose Reports
97
SURNAME FIRSTNAME
DOSIMETRIC SUMMARY
Name : SURNAME
First Name : FIRSTNAME From : 01/07/2015 to : 31/07/2015
Year : 2015 Dose Hp(10)G / Hp(10)N / Hp(0.07)
Month : 7
01/07/2015 5 / 0 / 0 µSv
02/07/2015 8 / 0 / 0 µSv
03/07/2015 28 / 0 / 0 µSv
06/07/2015 4 / 0 / 0 µSv
08/07/2015 11 / 0 / 0 µSv
09/07/2015 14 / 0 / 0 µSv
10/07/2015 15 / 0 / 0 µSv
13/07/2015 9 / 0 / 0 µSv
14/07/2015 7 / 0 / 0 µSv
15/07/2015 17 / 0 / 0 µSv
16/07/2015 2 / 0 / 0 µSv
17/07/2015 9 / 0 / 0 µSv
20/07/2015 10 / 0 / 0 µSv
21/07/2015 6 / 0 / 6 µSv
22/07/2015 9 / 0 / 12 µSv
23/07/2015 7 / 0 / 0 µSv
27/07/2015 10 / 0 / 0 µSv
28/07/2015 6 / 0 / 0 µSv
30/07/2015 17 / 0 / 18 µSv
31/07/2015 10 / 0 / 0 µSv
Sum : 204 / 0 / 36 µSv
Total dose for month : 7
204 / / 36 µSv
Total dose for year : 2015 787 / 0 / 264 µSv
SURNAMETotal dose for requested dates: / / µSv
Licensed to : MED TECH / SCGH
19/08/2015 1 / 49