Transcript ppt - Geant4 at SLAC

GEANT4 for dosimetric
study of an intracavitary
brachytherapy applicator
Emily Poon
Frank Verhaegen
March 6, 2006
McGill University
Montreal, Canada
Ir-192 HDR Brachytherapy
3.5 mm
1.1 mm
0.6 mm
5 mm
2m
Nucletron HDR ‘classic’ model
We generated a phsp file for
40 million photons reaching
the capsule surface in a
vacuum.
2
Validation of TG-43 parameters
1.0
1.1
1.0
0.9
0.9
0.8
F(r,theta)
radial dose function g(r)
(b)
0.8
0.7
0.7
0.6
GEANT4
0.6
0.5
Williamson and Li (1995)
0.5
r = 0.25 cm
r = 0.5 cm
r = 1 cm
r = 2 cm
r = 3 cm
r = 5 cm
0.4
0
2
4
6
8
10
12
radial distance (cm)
14
Radial dose function
Agreement: within 0.5%
0
30
60
90
120
150
180
theta (degrees)
Anisotropy function
Agreement: within 2 %
3
Modeling 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
4
Applicator with balloon
water or
contrast medium
tumor
•
•
protection for healthy tissue
contrast medium for dose
reduction and better
localization of balloon
balloon with
iodine solution
5
Plato treatment planning system
 CT-based
 dose calculations
according to TG-43
 assumes
homogeneous water
medium
 does not account for
applicator and
patient anatomy
6
TG-43 vs dose
kernel calculations
TG-43
Dose kernel
dose kernel:
• 100x100x40 voxels
(10x10x10 cm3)
• computed using DOSXYZ
because GEANT4 is too slow
Lead shielding
7
3-D patient calculations using dose kernels
50 %
100 %
300 %
no shielding
lead shielding
8
GEANT4 simulations
• Low energy model
• Photon transport only
• Kerma calculations using track length estimation
9
Dose around the tip region
no shielding
tungsten shielding
10
Dose around the balloon
no shielding
tungsten shielding
11
Experimental validation
Solid lines: GEANT4
dotted lines: EBT
no shielding
tungsten shielding
Good agreement between GEANT4 and GafChromic EBT film measurements
12
no shielding
tungsten shielding
• conformal distributions can be attained by proper selection
of source positions and dwell times
• tungsten shielding offers significant radiation protection
13
Ion chamber measurements
Extradin A14P chamber
192Ir
source
variable thickness
30 x 30 x 30 cm3 Lucite phantom
14
GEANT4 vs ion chamber measurements
DoseW/DosenoW
Ion chamber
GEANT4
• Ion chamber: high uncertainties in partially shielded regions
15
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!
16
Boundary crossing problems

We use track-length estimator
for kerma calculations
g(r) of an isotropic
 Dose dependent on photon
 Errors in calculations
 Error is larger when θ spans a
smaller angle
1.05
1.00
0.95
0.90
g(r)
When voxels are constructed
as segments of a sphere,
some photons cross the
boundaries without stopping
Ir point source
1.10
step size

192
0.85
0.80
∆θ = 1 deg
0.75
∆θ = 3 deg
0.70
∆θ = 180 deg
0.65
0.60
0
3
6
9
radial distance (cm)
12
15
17
User code…
We have an isotropic point source originating from (0,0,0)
Phantom is homogeneous water
Voxels are constructed as shown below:
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);
}
In this case, all photons reaching the voxel regions should have θ
between 87.5º and 92.5º.
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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
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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.
20