Referensdosimetri
Crister Ceberg
Medical Radiation Physics
Lund University
Sweden
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
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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 the
instruments in absorbed dose
to 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)
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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
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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
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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
Increased
temperature [K]
Specific heat capacity
[J kg-1 K-1]
Calorimeter
Ross and Klassen, PMB 41 (1996)
Calorimeter
Medin, Lund University
Calorimeter
AC
R
Variable R
B
Variable C
R
A
DU = UA-UB
Thermistors
Medin, Lund University
Calorimeter
Bridge output voltage(V)
6
post-irradiation drift
4
2
0
DV
-2
-4
-6
pre-irradiation drift
-8
0
60
120
180
240
300
360
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
Degree of equivalence
http://kcdb.bipm.org/
Uncertainty in Dw (1SD) at PSDL
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BIPM
NIST
NRC
METAS
PTB
LSDG
NMi
ARPANSA
BEV
ENEA
LNHB
(ion chamber)
(water calorimeter)
(water calorimeter)
(water calorimeter)
(water calorimeter)
(water calorimeter)
(graphite calorimeter)
(graphite calorimeter)
(graphite calorimeter)
(graphite calorimeter)
(graphite calorimeter)
0.30%
0.35%
0.41%
0.41%
0.20%
0.66%
0.40%
0.20%
0.37%
0.44%
0.47%
ND,W-based formalism
Dw,Q0 =MQ0 ND,w,Q0
• 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
Correction for radiation quality
Dw,Q =MQ ND,w,Q=MQ ND,w,Q0 kQ,Q0
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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
The beam quality correction factor
ND,w,Q Dw,Q /MQ
kQ,Q0=
=
ND,w,Q0 Dw,Q0/MQ0
• Most often, the reference beam quality is 60Co,
and then kQ,Q0=kQ
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
100
90
80
Relativ dos
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
Djup i vatten (mm)
35
40
45
50
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 detector
is sent for calibration at the standards laboratory
Practical details during measurement
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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 restabilisation (> 20 min), consider using normalisation
to 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
MQ=M kTP kelec kpol ks
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Uncorrected reading: M
Temperature and pressure: kTP
Electrometer correction factor: kelec
Polarity effect: kpol
Recombination effect: ks
Humidity: kh (rarely used)
Correction of MQ for influence quantities
MQ=M kTP kelec kpol ks
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Uncorrected reading: M
Temperature and pressure: kTP
Given by the SSDL
Electrometer correction factor: kelec
together with ND,w
Polarity effect: kpol
Recombination effect: ks
Not needed if calibrated
Humidity: kh (rarely used)
at 50% humidity (20-80%)
Temperature and pressure
• The temperature and pressure correction converts the cavity
air mass to the reference conditions:
(273.2+T) P0
kTP=
(273.2+T0) P
where the reference conditions generally are
– T0=20 C
– P0=101.3 kPa
Polarity
• Polarity correction for asymmetric charge collection
(mean value of readings at both polarities):
|M+|+|M-|
kpol=
2M
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
Recombination
• The recombination factor corrects for incomplete charge
collection using the ”two-voltage” method:
M1
M1
ks=a0+a1
+ a2
M2
M2
( ) ( )
2
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
Reference dosimetry in the user beam
• The absorbed dose to water is then given by
Dw,Q =MQ ND,w,Q0 kQ,Q0
where MQ is corrected for influence quantities,
and kQ,Q0 is determined based on the user’s beam quality
Reference dosimetry in the user beam
• Take care that the linac is operating properly!
• If not, accurate reference dosimetry
can be a source of errors