Post on 13-Jan-2016
description
GEANT4 for dosimetric GEANT4 for dosimetric study of an intracavitary study of an intracavitary brachytherapy applicatorbrachytherapy applicator
Emily PoonEmily PoonFrank VerhaegenFrank Verhaegen
March 6, 2006March 6, 2006
McGill UniversityMcGill UniversityMontreal, CanadaMontreal, Canada
22
Ir-192 HDR BrachytherapyIr-192 HDR Brachytherapy3.5 mm
5 mm
1.1 mm
2 m
0.6 mm
Nucletron HDR ‘classic’ model
We generated a phsp file for40 million photons reaching the capsule surface in a vacuum.
33
Validation of TG-43 parametersValidation of TG-43 parameters
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 30 60 90 120 150 180theta (degrees)
F(r
,th
eta)
r = 0.25 cm r = 0.5 cm r = 1 cm
r = 2 cm r = 3 cm r = 5 cm
Radial dose function Anisotropy function
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0 2 4 6 8 10 12 14radial distance (cm)
rad
ial
do
se f
un
ctio
n g
(r)
GEANT4
Williamson and Li (1995)
(b)
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0 2 4 6 8 10 12 14radial distance (cm)
rad
ial
do
se f
un
ctio
n g
(r)
GEANT4
Williamson and Li (1995)
Agreement: within 0.5% Agreement: within 2 %
44
Modeling of rectal applicatorModeling of rectal applicator
2 cm • made of silicone rubber• 8 catheters for HDR 192Ir source• allows for insertion of shielding
shielding made of lead or tungsten
55
Applicator with balloonApplicator with balloon
water or contrast medium
• protection for healthy tissue• contrast medium for dose reduction and better localization of balloon
tumor
balloon withiodine solution
66
Plato treatment planning systemPlato treatment planning system
CT-based CT-based dose calculations dose calculations
according to TG-43according to TG-43 assumes assumes
homogeneous water homogeneous water mediummedium
does not account for does not account for applicator and applicator and patient anatomypatient anatomy
77
TG-43 vs dose kernel calculations
Lead shielding
dose kernel:
• 100x100x40 voxels (10x10x10 cm3)• computed using DOSXYZ because GEANT4 is too slow
TG-43
Dose kernel
88
no shielding lead shielding
3-D patient calculations using dose kernels
50 %100 %300 %
99
GEANT4 simulationsGEANT4 simulations
• Low energy model • Photon transport only• Kerma calculations using track length estimation
1010
Dose around the tip regionDose around the tip region
no shielding tungsten shielding
1111
Dose around the balloonDose around the balloon
no shielding tungsten shielding
1212
Experimental validationExperimental validation
Good agreement between GEANT4 and GafChromic EBT film measurements
no shielding tungsten shielding
Solid lines: GEANT4dotted lines: EBT
1313
no shielding tungsten shielding
• conformal distributions can be attained by proper selection of source positions and dwell times• tungsten shielding offers significant radiation protection
1414
Ion chamber measurementsIon chamber measurements
Extradin A14P chamber
192Ir source
variable thickness
30 x 30 x 30 cm3 Lucite phantom
1515
GEANT4 vs ion chamber measurementsGEANT4 vs ion chamber measurements
Ion chamber GEANT4
• Ion chamber: high uncertainties in partially shielded regions
DoseW/DosenoW
1616
Speed issuesSpeed issues
Number of voxelsNumber of voxels Time/history (ms)Time/history (ms)
125125 0.8150.815
10001000 2.232.23
125000125000 120120
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!
1717
Boundary crossing problemsBoundary crossing problems We use track-length estimator We use track-length estimator
for kerma calculationsfor kerma calculations Dose dependent on photon Dose dependent on photon
step size step size When voxels are constructed When voxels are constructed
as segments of a sphere, as segments of a sphere, some photons cross the some photons cross the boundaries without stoppingboundaries without stopping Errors in calculationsErrors in calculations Error is larger when Error is larger when θθ spans a spans a
smaller anglesmaller angle
g(r) of an isotropic 192Ir point source
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
1.10
0 3 6 9 12 15
radial distance (cm)
g(r
)
∆θ = 1 deg
∆θ = 3 deg
∆θ = 180 deg
1818
User code…User code…
void Sphere01VoxelParameterisation::ComputeDimensions(G4Sphere& voxel,const G4int copyNo, const G4VPhysicalVolume*) const{ voxel.SetInsideRadius(rInner[copyNo]); voxel.SetOuterRadius(rOuter[copyNo]); voxel.SetStartPhiAngle(0.*deg); voxel.SetDeltaPhiAngle(360.*deg); voxel.SetStartThetaAngle(87.5*deg); voxel.SetDeltaThetaAngle(5.*deg);}
We have an isotropic point source originating from (0,0,0)
Voxels are constructed as shown below:
In this case, all photons reaching the voxel regions should have θ between 87.5º and 92.5º.
Phantom is homogeneous water
1919
User code (cont’d)User code (cont’d)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
G4ThreeVector preStepPos=aStep->GetPreStepPoint()->GetPosition();G4ThreeVector postStepPos=aStep->GetPostStepPoint()->GetPosition();
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
2020
ConclusionsConclusions 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.