Applications of Monte Carlo simulations to radiation dosimetry

Applications of Monte Carlo simulations
to radiation dosimetry
D.W.O. Rogers
Carleton Laboratory for
Radiotherapy Physics.
Physics Dept,
Carleton University,
Ottawa
http://www.physics.carleton.ca/~drogers
ICTP, Trieste, Nov 14, 2007
1

Papers in PMB and Med Phys with
Monte Carlo in title or abstract
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Radiation dosimetry in radiotherapy
• primary standards
– air kerma,
– absorbed dose
• electron & photon beams
• beta-ray fields
• clinical dosimetry protocols
– dose in a water tank
• TG51, TG61, TG43, TRS-398
• radiotherapy treatment planning
– dose in a (CT) patient
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adiation dosimeters
• many types of radiation dosimeters for radiotherapy
–ion chambers-the work horse for clinical reference
dosimetry and air kerma primary standards
– calorimeters for absorbed dose primary standards
– free air chambers for x-ray air kerma standards
–TLDs LiF
– diodes, MOSFETS
– radiographic and radiochromic films
– chemical (Fricke) dosimeters
Monte Carlo calculations have been
used to elucidate all of these.
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Farmer ion chamber
Ion chambers
from John McCaffrey, NRC
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Cavity theory: stopping-power ratios
Relates dose in cavity to dose in medium.
med
gas
sprs are fundamental to
-dosimetry protocols
-primary standards
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What is (L/ρ)?
A Spencer-Attix spr - stopping-power ratio
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Dosimetry in a water tank
P wall , P gr , P fl , P cel all 1% or less effects
-major variation comes from spr
for complete definitions of P wall etc see
http://www.physics.carleton.ca/~drogers/pubs/papers/ss96.pdf
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Electron beam depth-dose curve
12 MeV
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sprs in electron beams
Ding et al, Med Phys 22(1995) 489-501
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Realistic electron beam sprs
BEAM code used to simulate realistic accelerator
beams
Ding et al Med Phys 22 (1995)489
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Effects of realistic sprs
Ding et al MP 22(1995)489
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How to use realistic sprs
David Burns noted:
changing d ref simplifies everything.
d ref = 0.6 R 50 - 0.1 (cm)
The basis of electron beam dosimetry in
IAEA TRS-398 and AAPM TG-51 clinical protocols
Burns et al MP 23(1996)383
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Realistic sprs: d ref=0.6R 50 - 0.1
Burns et al MP 23(1996)383
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Photon beams: specifying beam quality
• NAP -nominal accelerating potential
• %dd(10) -percentage depth dose at 10 cm depth
in a 10x10 cm2 field on surface at SSD
100 cm
• %dd(10) X -the photon component of %dd(10)
(i.e., ignoring electron contamination)
• TPR20 10 -ratio of absorbed doses at depths 20
and 10 cm in a water phantom, measured
with a constant source-chamber
distance of 100 cm and a field size of
10x10 cm2 TG-51
TRS-398
at the plane of the chamber
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sprs for photon beams
filled: heavily filtered
open: lightly filtered
Kalach and Rogers 30 (2003) 1546-1555
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sprs for photon beams
filled: heavily filtered
open: lightly filtered
Kalach and Rogers 30 (2003) 1546-1555
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What happens without a flattening filter?
For IMRT, flattening
filter is not needed
(Titt et al, Med Phys
33(2006) 3270).
A single fit handles
both sets of beams
using %dd(10) x .
Major effect is on
%dd(10) x due to
non-flat beams
Xiong and Rogers, in prep, 2007
Based on full BEAM simulations.
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Two sets of
k Q values will
be needed,
one for with
flattening
filters, one
for machines
without them.
Flattening filter free: TPR
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Summary: protocol dosimetry
• the major quantity which varies in protocol
dosimetry is the stopping power ratio
– hence the discussion of it
• but other aspects of protocols such as TG-51 and
TRS-398 which are based on MC calculated values
–Pwall for plane parallel chambers in Co-60 beams
–Pcel for aluminium electrodes
– relationship between I50 and R50 in e- beams
• plus on-going research on other aspects
–Pwall for all beams, Prepl, effective point of
measurement
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Primary standards of air-kerma in Co-60
Primary standards in Co-60 beams are based on
cavity ion chambers and S-A cavity theory
D gas D wall /D gas D air /D wall
for complete definitions see
http://www.physics.carleton.ca/~drogers/pubs/papers/fundamentals_ss90.pdf
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How accurately can we calculate ion
chamber response?
Fano cavity chamber, - walls and gas the same material
(assume graphite) with a density ratio of about 1000.
- establish kerma to graphite in a parallel 60 Co beam.
Fano’s theorem => no fluence correction (traditionally
ignored, but in principle needed). All other K = 1.00
ie we can check our D gas calculation
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How accurately can we calculate ion
-cover of
EGSnrc
manual
-against
own cross
sections
-ESTEPE
is max
fractional
step size
chamber response? (cont)
This is the toughest test I know for any
electron-photon Monte Carlo code
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How accurately can we calculate ion
against
measured
data
chamber response? (cont)
Kawrakow & Rogers, MC2000, p135 based on data of
Nilsson et al, IAEA Proceedings, 1988
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K wall: attenuation and scatter
K air eqn ignores attenuation and scatter in chamber walls
Monte Carlo K wall scores
D gas without / D gas with scatter and attenuation
or
Or regenerate interacting photons &
ignore scattered photons
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K wall: non-linear extrapolation
Rogers & Bielajew, PMB 35 (1990) 1065
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Some measured confirmations of MC K wall
rotate the
chamber in Co-60
response*K wall
=response/A wall
should be constant
graphite walled chamber at NRC
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If A wall is
correct,
R/A wall should
be constant.
It is, within
0.3% despite
8% variation.
(residual 0.3%
is a K an effect)
Response vs angle of Mark IV
McCaffrey et al PMB 49(2004) 2491
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adial
PTB/OMH: cylindrical chamber
axial
45
axis of
rotation
measured response
vs wall thickness.
Should all
extrapolate to
same value.
Only the calculated K wall correction gave a constant response
Büermann et al PMB 48 (2003) 3581
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K an: axial non-uniformity
Bielajew developed an analytic theory to account for point
sources not parallel beams (PMB 35(1990)501 & 517)
A brute force MC calculation with a parallel beam or a point
source, confirms the analytic theory.
The corrections are all very small for Co-60 sources at 1 m
from typical chambers
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Revision of air-kerma standards
Using EGSnrc calculated K wall and K an values, revise the
reported values
Rogers and Treurniet, 1999 (NRC Report
PIRS-663)
extending work of
Bielajew and Rogers, PMB 37(1992)1283
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Revision of air-kerma standards (cont)
Note: the BIPM
baseline moved up
by 0.3%.
------
Monte Carlo =>
world’s air kerma
standards
increased 0.8%
(double stated
uncertainty)
Rogers & Treurniet
1999 NRC Report
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How accurate are calculations?
If we are going to use Monte Carlo calculated factors, we
need to know their uncertainty
How sensitive are they to:
-algorithm/computer code used
-cross sections
-spectrum used
-size of source
Rogers & Kawrakow Med Phys 30 (2003)521
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Calculated response of NRC 3C chamber
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K wall for NRC 3C
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(L/ρ) for different algorithms
EGSnrc
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K wall vs incident spectrum
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K an vs incident spectrum
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spr vs incident spectrum
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K an for 3C vs source radius
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Uncertainty estimates (%)
spr K wall
K an K comp
Stats

Verification of cavity theory?
Can Monte Carlo verify the accuracy of cavity theory?
EGSnrc can calculate D gas to 0.1%
(proof: Fano cavity calculations)
Cavity theory assumes that photon
interactions in the cavity do not occur
But Ma and Nahum showed they did.
PMB 36(1991)413
So does cavity theory hold for Ir-192 or lower energy
photon beams?
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Accuracy of Spencer-Attix cavity theory
spectrum
Another thought/computational experiment
For a parallel beam incident on a
stemless chamber filled with dry air
CAVRZnrc
SPRRZnrc
DOSRZnrc
EGSnrc
CAVRZnrc
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Accuracy of Spencer-Attix cavity theory
Only this
good
because
graphite
and air so
similar.
Calculations used ∆ = 10 keV for spr. Using
larger values brings value within 0.1% of unity
Borg et al, Med Phys 27(2000)1804
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The use of silicon diode detectors
• a common assumption is that diode detectors
measure dose directly
– ie no spr correction etc
• but sprs actually change quite a bit as the beam
quality changes
• Why don’t we need to correct for this?
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model of diode detector (Scanditronix EFD)
McKerracher and Thwaites
Radioth Oncol 79(06) 348
Wang Med Phys 34 (2007) 1734
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water/silicon stopping powers are not constant
calculate ratio
of dose in small
active region of
diode detector
isolated from
rest of detector
to dose to water
at same location.
Use CSnrc which
uses correlated
sampling
Wang Med Phys 34 (2007) 1734
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dose water/dose silicon active region
Wang Med Phys 34 (2007) 1734
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effect of backscatter from rest of chip
Wang Med Phys 34 (2007) 1734
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diode response at d max vs field size
mostly a
change in spr
effect as
d max changes
Wang Med Phys 34 (2007) 1734
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Summary re diode detectors
• diodes measure dose directly within +-1% as a
function of depth and beam quality in electron
beams
–one exception - small radius electron beams
• the silicon backing of the active region and the
epoxy play an important role in the flat
response
51/64

P TP: the pressure-temperature correction
t
ion
chamber
e-
for ion chambers
PTP is constructed so
independent of ρ
So E dep (ρ) is proportional to the density ρ.
E dep (ρ ο ) is independent of the density ρ.
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e-
ion
chamber
P TP (cont)
What happens if the electron
does not cross the cavity?
independent of ρ
E dep (ρ) is no longer proportional to the density ρ.
Hence the standard P TP correction factor may no longer work.
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Pressure vs. altitude
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NE2571 A12
“A4”
NRC xray
monitor
• EGSnrc Monte Carlo code
• cross-sections for DRY air
of different densities
• calculate D cav (dose to air)
• standard P TP correction
inherent in results
• PTB catalogued spectra
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Thimble chamber calculations
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Conclusions of P TP paper I
• there is a significant breakdown of the standard P TP
correction for low energy photon beams
• basic cause: e- stopping in the cavity, not crossing
• magnitude of the effect depends on:
– mismatch of wall to air cross sections
– fraction of dose due to photon interactions in the
cavity air
• a similar effect was reported in 2005 by the UW
ADCL for well ion chambers for I-125
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Experiments at NRC to demonstrate the effect
with Malcolm
McEwen
complete BEAMnrc model to give x-ray spectrum
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A variety of chambers studied
A2 NE2571
A12 NE2505
C552
aluminium
A19
C552 graphite
dural, C552
Kawrakow’s egs_view
Calculations with cavity.cpp, using Kawrakow’s
C++ geometry package & interface to EGSnrc
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Closed symbols:
P TP corrected
measured
responses
open symbols:
calculated
responses
Farmer-like chambers: 60 kV
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CAVRZnrc
uses a
cylindrical
model
cavity.cpp
includes the
conical end.
Effects of geometry details
These geometry differences have no effect in a
Co-60 beam
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Summary re P PT corrections
• measurements confirm the calculated breakdown
of the PTP correction factor for low-energy x-rays
• EGSnrc is capable of reproducing air-kerma
calibration coefficients well within 1%
–NKvs beam quality curves allow quantification
of the size of impurity effects
• geometry details have some effects at these low
energies although not at Co-60
• impurities are important at low photon energies
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Summary
MC techniques play a fundamental role in radiation dosimetry
• sprs and other corrections for ion chambers used
in clinical dosimetry
• correction factors for primary standards
• verification of cavity theory accuracy
• elucidation of detector response (eg diode)
• investigation of pressure-temperature effects
• and much, much more
– TLDs, OSL, alanine,Fricke, well chambers,
brachytherapy dosimetry etc
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Acknowledgements
• The work described here has been done in conjunction
with many colleagues, grad students and research
associates, without whom it wouldn’t get done.
• the various works described involved: Iwan Kawrakow,
David Burns, George Ding, Guoming Xiong, Nina Kalach,
Jette Borg, Alex Bielajew, John McCaffrey, Joanne
Truerniet, Lilie Wang and Dan La Russa, but many more
were involved in the overall project of Monte Carlo in
radiation dosimetry
• Support from the Canada Research Chairs program and
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