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Referensdosimetri Crister Ceberg Medical Radiation Physics Lund University Sweden
Referensdosimetri
Crister Ceberg
Medical Radiation Physics
Lund UniversitySweden
Reference dosimetry
• Determination of absorbed dose to water under reference conditions
• Not accounting for uncertainties related to non-reference conditions (for instance the patient...)
• High accuracy is needed (~ 1.5%)
• Requires a rigorous dosimetry protocol
Reference dosimetry protocols
• AAPM TG-51 (1999), North America
• DIN 6800-2 (1997, revised 2008), Germany
• NCS (1997), The Netherlands
• IPEMB (1996), UK
• IAEA TRS-398 (2000)
IAEA TRS-398
• Published in 2000
• Replace previous protocol TRS-277
• Based on calibration of theinstruments in absorbed doseto water
IAEA TRS-398
http://www-naweb.iaea.org/NAHU/DMRP/codeofpractice.html
International measurement system
Traceability
• Bureau International des Poids et Mesures (BIPM) 1875– International Laboratory for SI units
• Primary Standard Dosimetry Laboratory (PSDL)– Widely acknowledged
– Highest metrological qualities
– Accepted without reference to other standards
– About 20 worldwide
• Secondary Standard Dosimetry Laboratory (SSDL)– Depends on calibration at a PSDL
• Clinical user at the hospital– Depends on calibration at SSDL or PDSL
Primary standard laboratory (PSDL)
• ISO definition of primary standard– “a standard that is designated or widely acknowledged as having
the highest metrological qualities and whose values is accepted without reference to other standards of the same quantity“
• This requires an absolute dosimeter,– ”that can be assembled and used to measure the absorbed dose
deposited in its own sensitive volume without calibration in a known field of radiation” (Attix)
Absolute dosimeters
• Ionization chamber (BIPM)
• Fricke dosimeter (PTB until 2006)
• Water calorimeter (NIST, NRC, PTB since 2006)
• Graphite calorimeter (NPL)
Ionization chamber
• Graphite ion chamber with well defined volume
• Designed to fulfill the Bragg-Gray conditions with very little perturbation
Ionization chamber
Boutillion and Perroche, PMB 38 (1993)
Ionization chamber
M/m
W
Boutillion and Perroche, PMB 38 (1993)
Fricke dosimeter
• Ferrous ions (Fe2+) in water
• Interaction with ionizing radiation leads radiolysis of water
• Iron ions are oxidized to ferric ions (Fe3+)
• The UV transmission spectrum is affected
• Response is determined by total absorption of electrons
Fricke dosimeter
Feist, PMB 27 (1982)
Calorimeter
• Measurable temperature increase in medium– Water
– Graphite
• Assumes that– All radiation energy is transfered to heat
– All heat comes from radiation energy
Calorimeter
• Measures directly the absorbed energy per unit mass
D=DTi·ci
Increasedtemperature [K]
Specific heat capacity[J kg-1 K-1]
Calorimeter
Ross and Klassen, PMB 41 (1996)
Calorimeter
Medin, Lund University
Calorimeter
B A
AC
Variable R
Thermistors
R
RVariable C
DU = UA-UB
Medin, Lund University
Calorimeter
0 60 120 180 240 300 360
-8
-6
-4
-2
0
2
4
6
DV
post-irradiation drift
pre-irradiation drift
Bri
dg
e o
utp
ut vo
lta
ge
(V
)
Time (s)
Medin, Lund University
Calorimeter
• Requires several correction factors
D=DTi·ci·kc·kp·kdd·(1-kHD)-1
kc: heat conductivity
kp: perturbation due to glass container
kdd: radiation field inhomogeneity
kHD: heat defect
Uncertainty in Dw (1SD) at PSDL
• BIPM (ion chamber) 0.30%
• NIST (water calorimeter) 0.35%
• NRC (water calorimeter) 0.41%
• METAS (water calorimeter) 0.41%
• PTB (water calorimeter) 0.20%
• LSDG (water calorimeter) 0.66%
• NMi (graphite calorimeter) 0.40%
• ARPANSA (graphite calorimeter) 0.20%
• BEV (graphite calorimeter) 0.37%
• ENEA (graphite calorimeter) 0.44%
• LNHB (graphite calorimeter) 0.47%
ND,W-based formalism
• Q0 denotes the reference beam quality at the standard laboratory
• DW,Q0 is known at the standard laboratory
• MQ0 is the dosimeter reading under reference conditions
• ND,W,Q0 is the dosimeter’s ”absorbed-dose-to-water” calibration coefficient
Dw,Q0 =MQ0 ND,w,Q0
Correction for radiation quality
• Q denotes the quality at the user
• DW,Q is the absorbed dose in the user’s beam
• MQ is the dosimeter reading at the user
• kQ,Q0 corrects for the effects of the difference between the reference beam quality and the beam quality at the user
Dw,Q =MQ ND,w,Q=MQ ND,w,Q0 kQ,Q0
The beam quality correction factor
• Most often, the reference beam quality is 60Co,and then kQ,Q0=kQ
ND,w,Q Dw,Q/MQ
ND,w,Q0 Dw,Q0/MQ0kQ,Q0= =
Experimental determination of kQ,Q0
• Ideally, the kQ,Q0 factor should be measured for each chamber at the desired beam quality
• However this requires– Standard laboratories with clinical beam qualities
– Independent dosimetry (e.g. calorimeters) operating at these beam qualities
Theoretical determination of kQ,Q0
• Instead, theoretically determined kQ,Q0 factors are tabulated in the IAEA TRS 398 protocol– for different chamber types
– for different beam qualities
0.2%
Beam quality
• For photons TPR20,10
– TPR20,10=TPR(20)/TPR(10)
– TPR20,10=1.2661 PDD20,10 – 0.0595
• For electrons R50
– Depth where absorbed dose is 50% of maximum
– R50=1.029 R50,ion - 0.06 (R50,ion ≤ 10 cm)
– R50=1.059 R50,ion - 0.37 (R50,ion > 10 cm)6 MeV elektoner
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45 50
Djup i vatten (mm)
Rela
tiv d
os
Practical details before use
• Store instruments in a safe place (dry, normal room temperature, clean, etc.)
• Inspect the instrument before use
• Make an x-ray image of the instrument at the time of purchase, or if problems arise
Stability check
• Check and document long term stability(both detector and electrometer)
• Perform stability check before and after the detectoris sent for calibration at the standards laboratory
Practical details during measurement
• Allow time to reach thermal equilibrium
• Turn on the electrometer at least 1-2 h before use
• Always collect several measurements (5-10)
• Pay attention to trends (can be sign of equipment failure)
• Leakage should be < 0.1% of MQ
Pre-irradiation effects
• Pre-irradiate the chamber to reach charge equilibrium in the exposed materials
• After change of voltage or polarity, be careful with re-stabilisation (> 20 min), consider using normalisationto an external monitor chamber
Pre-irradiation effects
• Normalized response for an NE 2571 to 60Co irradiation
McCaffrey et al. PMB 2005
Correction of MQ for influence quantities
• Uncorrected reading: M
• Temperature and pressure: kTP
• Electrometer correction factor: kelec
• Polarity effect: kpol
• Recombination effect: ks
• Humidity: kh (rarely used)
MQ=M kTP kelec kpol ks
Correction of MQ for influence quantities
• Uncorrected reading: M
• Temperature and pressure: kTP
• Electrometer correction factor: kelec
• Polarity effect: kpol
• Recombination effect: ks
• Humidity: kh (rarely used)
MQ=M kTP kelec kpol ks
Given by the SSDLtogether with ND,w
Not needed if calibratedat 50% humidity (20-80%)
Temperature and pressure
• The temperature and pressure correction converts the cavity air mass to the reference conditions:
where the reference conditions generally are– T0=20 C
– P0=101.3 kPa
(273.2+T) P0
(273.2+T0) PkTP=
Polarity
• Polarity correction for asymmetric charge collection(mean value of readings at both polarities):
where M+ and M- are the readings at positive and negative polarity, respectively
• If not performed at the standards laboratory,a correction for the relative effect can be done
|M+|+|M-|2M
kpol=
Recombination
• The recombination factor corrects for incomplete charge collection using the ”two-voltage” method:
where M1 and M2 are the readings at two different operating voltages V1 and V2, such that V1 ≥ 3 V2,and a0 - a2 are constants found in the TRS 398 protocol
• If not performed at the standards laboratory,a correction for the relative effect can be done
M1
M2
ks=a0+a1 + a2( ) M1
M2( )
2
Reference dosimetry in the user beam
• The absorbed dose to water is then given by
where MQ is corrected for influence quantities,and kQ,Q0 is determined based on the user’s beam quality
Dw,Q =MQ ND,w,Q0 kQ,Q0
Reference dosimetry in the user beam
• Take care that the linac is operating properly!
• If not, accurate reference dosimetrycan be a source of errors
