Dosimetry - SASRO€¦ · Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 16 Absolute / relative...

October 19th 2009 Dosimetry – A. Mack & M. Sassowsky

Dosimetry

PD Dr.Dr. Andreas MackKlinik Hirslanden

Institute for Radio-Oncology – RTAG&

Dr. Manfred Sassowsky (manuscript 2007)Cantonal Hospital Lucerne (KSL), Institute for Radio-Oncology

19.10.2009

Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 1

Dosimetry

Dr. Manfred Sassowsky

Cantonal Hospital Lucerne (KSL)

Institute for Radio-Oncology

3.9.2007

• Introduction

• Metrological traceability; the dosimetry chain

• Absolute / relative dosimetry

• Ionisation chambers

• Thermoluminescent Detectors (TLD)

• Film dosimetry

• Portal dosimetry

• Small field dosimetry

KantonsspitalLuzern


Literature

1. E.B. Podgorsak (Technical Editor): Radiation Oncology Physics: A

Handbook for Teachers and Students, IAEA, Vienna, 2005, ISBN 92–0–

107304–6, http://www.iaea.org/books

2. TRS 398: Absorbed Dose Determination in External Beam Radiotherapy,

IAEA, Vienna, 2000

3. H. Reich (Hrsg.): Dosimetrie ionisierender Strahlung, B.G. Teubner,

Stuttgart, ISBN 3-519-03067-5 (out of print)

4. H. Krieger: Strahlenphysik, Dosimetrie und Strahlenschutz (2 volumes),

B.G. Teubner, 2001, ISBN 3-519-23078-X and ISBN 3-519-43052-5

5. Recommendations of the Swiss Society of Radiobiology and Medical

Physics (http://www.sgsmp.ch/recrep-m.htm#rec)


Introduction (1)

• Dosimetry = dose measurement

- Dose, here: amount of radiation

- Metrology: science and technique of measurement

(... not to be confused with meteorology ...)

• A dosimeter is a device that measures (directly or indirectly)

- Exposure

- Kerma

- Absorbed dose

- Equivalent dose

- or other related quantities

• Dosimeter system = detector + reader (+ auxiliary equipment)

e.g.: Ionisation chamber + electrometer + check source


Introduction (2)

• Absorbed dose is the deposited energy per mass:

• SI unit is the gray (Gy):

• Dose rate ist the absorbed dose per unit of time:

• SI unit is the gray per second (Gy/s):

• Water is commonly used as reference material

(Properties similar to tissue, availability, physical properties well defined)

dm

dED =

Gykg

J===

][

][][

m

ED

kg

J1 Gy1 =

dt

dDD =•

s

Gy==

•

][

][][

t

DD


Introduction (3)

• Ideally a dosimeter system should have the following properties:

- High accuracy and reproducibility

- Linearity of signal with dose

- Adequate spatial resolution

- Large dynamic range

- Small dependence of signal on

o Dose rate

o Beam quality

o Direction

• Not all requirements can be fulfilled by a single dosimeter system

• For a given application, the most suitable system must be chosen


Introduction (4)

• Accuracy: Proximity of measured values to the “true” value

• Reproducibility: Degree of agreement between repeated measurements

• Our “Target”: measure the “true value”

• Accuracy versus reproducibility:

Reproducibility High High Low Low

Accuracy High Low High Low


Introduction (5)

• The „true value“ is not known

• Accuracy and reproducibility of a measurement are expressed by its

Measurement uncertainty

• ISO standard „Guide to the expression of uncertainty in measurement“ (GUM)

- Procedure for characterizing the quality of a measurement

- Generally accepted in many fields

- Defines uncertainty as a quantifiable attribute of a measurement


Introduction (6)

• Ionising radiation can not be measured directly -

- but only by its interaction with matter

• Different types of fundamental interactions

have been treated in the lecture “Basic radiation physics”

• Dosimetry methods presented in this lecture:

Methods... Physical effect ...

- Calorimetry Heating of water

- Ionisation chambers Ionisation of air

- TLDs Excitation of energy levels in crystals

- Film dosimetry Ionisation of AgBr crystals in radiographic film


Metrological traceability: ... What‘s that ???

• Dose prescription in PTV, e.g.: 30 × 2 Gy = 60 Gy

• How do you know that the delivered dose per fraction is indeed 2 Gy ?

• The Linac displays MU (monitor units), this must be calibrated.

• How and against what ?

=> Against a dosimeter (ionisation chamber) in a phantom

• This dosimeter must also be calibrated

• How and against what ?

• ....


Metrological traceability: The metrological „Pyramid“

National standards

Primary

Secondary

Local reference

standards

Working standards

Field instruments

e.g. for MV photons:

Water calorimeter

Ionisation

chambers

Dosimeter system

METAS

Radio-oncology

departments

Sent to METAS for verification

at least every 4 years

Traceability: The result of a measurement can be related to a (primary) standard

through an unbroken chain of calibrations all having stated measurement uncertainties.


Metrological traceability: Intercomparisons

• Primary standards are compared internationally

with other primary standards

• Interlaboratory comparisons with several

participants or bilaterally

• Assure that the primary standards agree within

their measurement uncertainties

• National dosimetry intercomparisons

organised by SSRMP

• TLDs sent to radio oncology

departments for irradiation

• Results evaluated centrally and

published in anonymised form


Metrological traceability: Primary standards

• Measurement setups with highest metrological quality, lowest uncertainty

• Measurement results deduced from "first principles",

or simple physical relations

• Can eventually be traced back to fundamental constants

• Can obviously not be calibrated

(as they are supposed to serve as the origin in the metrological pyramid)

• Require significant time and effort; not suitable for clinical environment

• Examples:

- Superficial X rays: parallel plate ionisation chamber

- MV photon beams: water calorimeter

- MV electron beams: Fricke dosimetry


Metrological traceability: Water calorimeter (1)

• Primary standard for MV photon beams

• Measures temperature increase caused by

deposited energy

NTC Glass capillary

Connection wires

Insulation

Epoxy resin

≈ 0.25 mm ≈ 0.5 mm

hdWWW k

cTD−

⋅⋅Δ=1

1

• Vessel with ultra-pure water and 2 miniature

temperature probes

• NTC: temperature dependent resistance

• Measured with bridge circuit

DW = Absorbed dose to water

ΔTW = Measured temperature increase

cW = Specific heat capacity of water

khd = Correction for heat defect



Heat exchangerThermal insulation

(styrofoam)Stirrer

Beam

Glass vessel

Thermistors

Water phantom

Monitor chambers

Air

Pt100 temperature probes

BeamGlass vessel

Thermal insulation (styrofoam)

Monitor chambers

Heat exchanger

• Vessel embedded in water phantom

• Temperature stabilised at 4oC

(maximum density of water)



-15

-10

-5

0

5

10

15

0 40 80 120 160 200 240 280 320 360

Zeit [s]

Brü

ck

en

sp

an

nu

ng

[ȝV

]

12.06

12.08

12.10

12.12

12.14

12.16

12.18

12.20

220 240 260 280 300 320 340 360

-11.56

-11.54

-11.52

-11.50

-11.48

-11.46

-11.44

-11.42

-11.40

-11.38

-11.36

-11.34

0 20 40 60 80 100 120

ǻU

m2, b2

m1, b1

U1 = m1 t + b1

U2 = m2 t + b2

Bri

dg

e v

olt

ag

e[μ

V]

Time [s]

• One calorimeter run

• Note:

ΔU ≈ 24 μV

ΔTW ≈ 1.2 mK

= 0.0012oC

• Typical measurement

series needs about 100

calorimeter runs

• Measurement

uncertainty (60Co):

ΔDW/DW =0.41% (k=1)


Absolute / relative dosimetry

• Absolute dosimetry

- Measurement of absolute dose

at a reference point on the central ray of a beam

- Accomplished using primary standards,

secondary standards and local reference standards

- Secondary and local reference standards: ionisation chambers

• Relative dosimetry

- Measurement of dose relative to reference point

o Depth dose curve on central ray of the beam

o Transverse dose distributions in different depths


Ionisation chamber: Principle

• Cavity filled with gas (usually air)

• Two electrodes on HV (U)

• Beam ionises air molecules

• Charge separation in electric field

• Current (I) => Dose rate

• Charge (Q=∫ I dt) => Dose

• Sensitive volume open to

ambient air (p, T vary)

• Variety of different shapes:

Beam

U

I


Ionisation chamber: Cylindrical (thimble) chamber

Insulator Outer electrode Central collecting electrodeHousing

• Most popular design

• Signal independent of radial beam direction

• Typical sizes:

- Length: 4 ... 25 mm

- Radius: 2 ... 7 mm

- Volume: 0.05 ... 1 cm3

• Thin walls: ~0.1 g/cm2

• Used for photon and electron beams

PTW Farmer chamber


Ionisation chamber: Parallel plate chamber

1 – Polarising electrode

2 – Collecting electrode

3 – Guard ring

Cut A-B

• Recommended for dosimetry of electron beams

• Useful for depth dose measurements

• Useful for measurements in build-up

region of MV photon beams

PTW Roos chamber


Ionisation chamber: Well type chamber

• Used for brachytherapy sources

(=> Lecture "Brachytherapy")

• High sensitivity

• Large volumes

• Can be designed to accommodate

various source sizes

PTW well type chamber

To electrometer

Source holder

Outer electrode

Collecting electrode


Ionisation chamber: Segmented chamber

• Many individual chambers arranged in an array

• Used to measure 2D dose distributions in a plane

• Application: IMRT quality assurance


Ionisation chamber: Electrometer

• Currents / charges to be measured are very low

(Currents in the range of 0.1 ... 100 nA; 1 nA= 10-9 A)

• Device to measure such low currents / charges:

electrometer

• Operational Amplifier with high input impedance

• Feedback with resistor for current measurement

• Feedback with capacitor for charge measurement

PTW Unidos E electrometer

R

Uin

Iin

Uout

R

UI out

in −=

+

-

Uin

Iin

Uout

C

-

+

CUtI outin ⋅−=⋅


Ionisation chamber: Dose determination

= Absorbed dose to water at beam quality Q

= Calibration factor at calibration beam quality Qc

= Correction factor for (eventual) difference between Q and Qc

= Corrected instrument reading at beam quality Q

QQQQWQW MkNDCC⋅⋅= ,,,

CQWN ,

CQQk ,

QM

QWD ,

C

Gy=][ , CQWN

C=][ QM

Gy=][ ,QWD

Favourable situation in CH:

• Qc very close to Q• => very close to 1

CQQk ,


Ionisation chamber: Correction factors

= Corrected instrument reading at beam quality Q

= Air density correction factor

= Ion recombination correction factor

= Uncorrected instrument reading at beam quality Q

STpQ kkMM ⋅⋅=

0

0

T

T

p

pkTp ⋅=

Tpk

Sk

QM

M

C=][ QM

C=][M

00 , Tp

Tp ,

= Pressure and temperature at reference conditions

= Pressure and temperature at measurement conditions

K hPa ==== ][][;][][ 00 TTpp


Ionisation chamber: Reference conditions (1)

MV Photons:

• Temperature T0 = 293.15 K = 20oC

• Pressure p0 = 1013.25 hPa

• Relative humidity 50 %

• Beam quality QC = TPR20,10 (calibration)

• Source chamber distance 100 cm

• Depth in water (d) 5 cm (60Co)

10 cm (MV Photons)

• Field size (50% isodose) 10 x 10 cm2 at depth d

SCD = 100 cm

d

Field size = 10 × 10 cm2



Calibration beam qualities QC available in CH for MV Photons



Electrons:

• Temperature T0 = 293.15 K = 20oC

• Pressure p0 = 1013.25 hPa

• Relative humidity 50 %

• Beam quality QC= R50 (calibration)

• Source surface distance 100 cm

• Depth in water (d) d = 0.6 R50 – 0.1 gcm-2

• Field size (50% isodose) 15 x 15 cm2 at phantom surface

SSD = 100 cm

d

Field size = 15 × 15 cm2



Calibration beam qualities QC available in CH for MV Electrons


Ionisation chamber: Advantages / Disadvantages

• High accuracy and reproducibility

• Necessary correction factors well understood

• Instant readout

• Finite measurement volume


Thermoluminescent detectors (TLD) Principle (1)

• Upon absorption of radiation, some materials retain part of the absorbed

energy in meta-stable states

• When this energy is subsequently released in the form of light, this

phenomenon is called luminescence

• Light may be ultraviolet, visible or infrared – depending on the material

• Two types of luminescence, distinguished by time delay between

stimulation and emission of light:

- Fluorescence: time delay 10-10 … 10-8 s

- Phosphorescence: time delay > 10-8 s

• Most commonly used materials in clinical dosimetry:

- LiF:Mg,Ti

- LiF:Mg,Cu,P

- Li2B4O7:Mn


TLD: Principle (2)

Irradiation

• Crystals contain impurities (type 1)

• They lead to meta-stable energy levels

(„Storage traps“)

• Upon irradiation, electrons are shifted to

conduction band

• They may either recombine directly ...

• ... or become trapped

Conduction band

Valence band

Storage trap

Direct

recombination

Ionising

radiation

Conduction band

Valence band

Storage trap

Heat

Light emission

Recombination center

Readout

• Crystals contain impurities (type 2)

• They facilitate recombination of electrons

with holes („recombination centers“)

• Upon heating, electrons are shifted to

conduction band

• They release light when they combine

with a hole at the recombination center


TLD: TLD reader

• Heater

• Photo-multiplier tube (PM)

- detects light from TLD

- converts it to an

amplified electrical signal

• Electrometer records PM signal

• Display of signal vs. temperature:

“Glow curve”

• Dose determined from area below peak

- Calibration

- Energy correction

- Non-linearity correction

- Fading


TLD: Applications

• In-vivo dosimetry

• Monitoring for radiation protection

• Dose distributions

• TLD intercomparisons organised by SSRMP


TLD: Advantages / disadvantages

• TLDs are available in various geometric shapes

• Can be made small in size => point dose measurements

• Many TLDs may be used in a single exposure

• Cheap

• No instant readout

• Readout time consuming

• Accurate results require careful calibration and handling,

as well as significant time and effort

• Signal erased during readout


Film dosimetry: Principle

Radiographic film:

• Base layer covered with a sensitive

emulsion of AgBr crystals in gelatine

• Irradiation: AgBr is ionised:

Ag+ ions are reduced to elementary Ag:

Ag+ + e- → Ag

• Ag is black and forms a „latent“ image

• Development: other Ag+ ions in one crystal

are reduced, if elementary Ag is present

• Fixation: rest of AgBr (in undeveloped

grains) is washed away

• => Permanent image of dose distribution

Coating

Base (typ. 200 μm)

Emulsion (10 … 20 μm)

Electron micrograph of AgBr grains in gelatine

Typical size 0.1 … 3 μm


Film dosimetry: Optical density

• Light transmission through the film is a function of the film opacity

• Can be measured in terms of Optical density (OD) with a densitometer

• Optical density is defined as:

⎟⎠⎞

⎜⎝⎛=

I

IOD 0

10log 0I

I= Initial light intensity

= Intensity transmitted through the film

• Relationship between dose and OD:

- „Not strictly“ linear

- Depends on film and processing

- Described by sensitometric curve

- Must be established before use of film for dosimetry


Film dosimetry: Sensitometric curve

Regions / parameters:

• Fog: OD of unexposed film

• Speed: exposure required to

produce an OD>1 over the fog

• Toe: transition to linear part

• Gamma: slope of the linear part

• Latitude: range of exposures

that fall in the linear part

• Shoulder: Saturation of OD for

high exposures

OD

Exposure

7

6

5

4

3

2

1Fog Toe

Linear part

Shoulder


Film dosimetry: Applications

• Portal imaging

• Qualitative dose measurements

• Quantitative dose measurements: need careful calibration, use and analysis

• Quality control of radiotherapy machines, e.g.:

- Congruence of light and radiation fields

- Dose profile at given depth in a phantom

- …

• Verification of treatment techniques in phantoms


Film dosimetry: Advantages / Disadvantages

• Film can be archived

• High 2D resolution

• Very thin: does not disturb beam

• Processing facilities (development, fixation) required

• Not trivial to achieve reproducible processing of the film

• Variation between films and production batches

• Quantitative dosimetry needs careful calibration

• Useful dose range of film is limited

• Energy dependence, in particular for lower photon energies


Film dosimetry: Radiochromic film

• More recent development: Radiochromic film

• Principle: contains dye that is polymerised

and develops a blue color upon exposure to radiation

• Self-developing, requires neither development nor fixation

• Sensitometric curve must be measured with densitometer

• Advantages with respect to radiographic film:

- No film processing => no quality control of film processing

- Grain-less material => higher resolution

- Can be used in regions with high dose gradients

- Energy dependence less pronounced

• Disadvantage: less sensitive than radiographic film


Portal dosimetry

• Dose measurement / imaging in treatment beam

- Verify treatment portals, compare with simulator radiographs

- Verify patient setup

• Traditional method: film dosimetry using dedicated film types

- Drawbacks:

o Image quality poor compared to conventional X ray images

o Requires time and effort

o Offline evaluation

• More recent development

and nowadays a standard:

EPID = Electronic portal imaging device


Portal dosimetry

EPID system consists of:

• Suitable radiation detector

- Fluoroscopic detector

- Segmented ionisation chamber

- Amorphous silicon detector

• Data acquisition system to transfer detector information to a computer

• Software to process information and convert it to an image


Portal dosimetry

• Amorphous Silicon Detector

• Array of typ. 200 000 pixels

• Pixel pitch: typ. 0.8 mm MV photon

Electron

a-Si Photodiode

Phosphor

Copper plate

Glass substrate

Light

FET transistors

for readout

• Cut through

one pixel:

Typ. 500 columns

Typ.

400

rows

Beam


Small field dosimetry

• Smallest “standard” photon fields have transverse field size down to

approximately 4 x 4 cm2

• Certain advanced radiotherapy techniques use the superposition of

multiple small fields, e.g.

- IMRT

- Stereotactic radiosurgery

(see lectures “Treatment planning systems”, “Special techniques”)

• Issues with small fields:

- Penumbra regions overlap

- Multiple steep gradients at individual field edges

- Modelling in treatment planning systems

- Dose measurement (small volumes / steep gradients)



• Overlapping of penumbra regions

• Transverse dose profile

(see lecture “Beam production”)Drel (%)

D0

100

50

xField size

Penumbra region




Drel (%)

x




Drel (%)

x




Drel (%)

x

Slight reduction of output factor




Drel (%)

x

Significant reduction of output factor



• Dose measurement (small volumes / steep gradients)

- Sensitive volume of detector should be small compared to field size

- Positioning of detector

- Disturbance of radiation field by detector

- No secondary electron equilibrium in transverse field direction

- …

• No “standard approach” yet; ongoing investigations / “research”

PTW pinpoint ionisation chamber

Vsens = 16 mm³

PTW silicon diode detector

Vsens : disc of 1mm2 area

PTW diamond detector

Vsens = 1 … 6 mm³



• Dose measurement in fluence modulated fields (IMRT):

- Film

- Segmented ionisation chamber



• Dose measurement in fluence modulated fields (IMRT): Gamma method

Reference distribution

Measured distribution

Position

Acceptance criteria

ΔDmax (e.g. 3 %)

Δdmax (e.g. 3 mm)

• γ < 1 =>

• γ > 1 =>

ΔDmax Δdmax

2max

2

2max

2

mind

d

D

D

ΔΔ

+ΔΔ

=γDose




Position

ΔDmax Δdmax

Dose






Position

ΔDmax Δdmax

Dose






Position

ΔDmax Δdmax

Dose




The end ...

• Thank you for your attention !

• Questions ?

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