GEANT4%20for%20dosimetric%20study%20of%20an%20intracavitary%20brachytherapy%20applicator

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GEANT4 for dosimetric study of an intracavitary
brachytherapy applicator

• Emily Poon
• Frank Verhaegen
• March 6, 2006

• McGill University
• Montreal, Canada

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Ir-192 HDR Brachytherapy
Nucletron HDR classic model
We generated a phsp file for40 million photons
reaching the capsule surface in a vacuum.
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Validation of TG-43 parameters
Radial dose function
Anisotropy function
Agreement within 0.5
Agreement within 2
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Modeling of rectal applicator

• made of silicone rubber
• 8 catheters for HDR 192Ir source
• allows for insertion of shielding

shielding made of lead or tungsten
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Applicator with balloon
water or contrast medium

• protection for healthy tissue
• contrast medium for dose
• reduction and better
• localization of balloon

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Plato treatment planning system

• CT-based
• dose calculations according to TG-43
• assumes homogeneous water medium
• does not account for applicator and patient
  anatomy

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TG-43 vs dose kernel calculations
dose kernel
TG-43

• 100x100x40 voxels
• (10x10x10 cm3)
• computed using DOSXYZ
• because GEANT4 is too slow

Dose kernel
Lead shielding
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3-D patient calculations using dose kernels
no shielding
lead shielding
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GEANT4 simulations

• Low energy model
• Photon transport only
• Kerma calculations using track length estimation

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Dose around the tip region
no shielding
tungsten shielding
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Dose around the balloon
no shielding
tungsten shielding
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Experimental validation
Solid lines GEANT4 dotted lines EBT
no shielding
tungsten shielding
Good agreement between GEANT4 and GafChromic EBT
film measurements
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no shielding
tungsten shielding

• conformal distributions can be attained by
  proper selection
• of source positions and dwell times
• tungsten shielding offers significant radiation
  protection

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Ion chamber measurements
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GEANT4 vs ion chamber measurements
DoseW/DosenoW
Ion chamber
GEANT4

• Ion chamber high uncertainties in partially
  shielded regions

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Speed issues
Number of voxels Time/history (ms)
125 0.815
1000 2.23
125000 120
CPU time for a 2.4 GHz processor to simulate a
photon history in a 30x30x40 cm3 water phantom
GEANT4 is too slow for patient calculations!
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Boundary crossing problems

• We use track-length estimator for kerma
  calculations
• Dose dependent on photon step size
• When voxels are constructed as segments of a
  sphere, some photons cross the boundaries without
  stopping
• Errors in calculations
• Error is larger when ? spans a smaller angle

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User code
We have an isotropic point source originating
from (0,0,0)
Phantom is homogeneous water
Voxels are constructed as shown below
In this case, all photons reaching the voxel
regions should have ? between 87.5º and 92.5º.
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User code (contd)

• We set the voxels as sensitive detector (SD)
• As a test in ProcessHits function of the
  user SD class, we
• recorded the pre-step and post-step positions
  of photons
• entering the voxels

• preStepPos.theta() and postStepPos.theta()
  should be
• between 87.5º and 92.5º
• In a test run of 1 million histories, 0.6 of
  the photons
• crossed the boundaries

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Conclusions

• Dosimetric properties of a novel intracavitary
• brachytherapy applicator have been studied.
• GEANT4 results are in good agreement with
• GafChromic EBT film and ion chamber
• measurements.
• A phsp file of the 192Ir source that we
• generated using GEANT4 will be used in
• another code (to be developed) for fast Monte
• Carlo calculations.
• Speed and some boundary crossing issues
• need to be addressed.