1. Geant4 Monte Carlo Characterization of Novel Electronic Brachytherapy System
Derek Liu
E. Poon, M. Bazalova, B. Reniers, M. Evans
J. Seuntjens, T. Rusch*, F. Verhaegen
McGill University
* Xoft, Inc.

2. Axxent Miniature X-ray Tube
Recently, Xoft Inc. has developed a novel Electronic Brachytherapy System…the Axxent. This system is unique in the sense that the radiation is generated by a miniature x-ray tube embedded in the tumor.
Things to note:
Water cooling catheter
Max 50 kVp

3. Project Objectives Characterize x-ray tube
Monte Carlo with experimental verification
Beam quality
HVL
Attenuation curve
Photon Spectrum
Spectrum
2-D Dose distribution
Gafchromic film in Lucite Phantom
I’ll be comparing three quantities: HVL, Spectrum, and dose distribution

4. Attenuation Curve NE 2571 farmer 2 mm Pb collimator 0.5 cm aperture
Src-collimator: 1”
Src-Al foil: 2”
Src-detector: 10”
So let’s go through the experimental setup…
Things to note:
Particularly difficult problems with xoft: output instability, output drift, src variation
Output fluctuation
Output monitor
Beam hardening
Chamber response
Scatter
Subtract scatter

5. Spectrum Amp-Tek CdTe 1 mm Crystal 4 mil Be Window 2 mm W collimator
2 mm aperture
Src-CdTe: 1.5 m
The spectrum experimental setup is similar to the attenuation curve…
Air attenuation
Attenuation correction
Detector response
Stripping procedure
Energy Calibration
31.0 keV Ba-133 decay
14.9 keV Yttrium K-alpha

6. Geant 4.8.1 Geometry Why use Geant? Tip thickness 0.2 mm
Complex geometry
Low energy physics
C++
Coding flexibility
Geometry
Tip thickness 0.2 mm
Transmission Tungsten target thickness 0.7 um
2 mm water
So I confess, I’m a Geant user…my supervisor and I chose Geant because…
Here is a simplified picture of my geometry, many more components are not included to ease of visualization. The electrons are generated…
Things to note:
Where the Tungsten is
Where the tip is
Where the water

7. Geant 4 Bremsstrahlung Production
Just to show you some physics comparisons in the energy range I’m interested in…
Things to note:
Comparing absolute spectra, G4 Brem is ~10% higher than BEAM
Bremsstrahlung mean free path
Bremsstrahlung angular distribution

8. Geant 4 Bremsstrahlung Production
As Dr. Ma mentioned, it is very easy to make a mistake coding in Geant4. And if you ask my McGill colleagues, they’ll tell you the story of a crucial mistake concerning 0.7um Tungsten. Here is a comparison of BEAMnrc and Geant4 simulations of a simplified geometry: slabs of Tungsten, AlN with Y, water
Things to note:
Yttrium K lines
Slight offset of K-beta energy
Brem production slight higher for G4
No Tungsten L lines

9. Source to Source Variation
Here are spectral measurements from 4 different sources…
Things to note:
Large variation

10. Comparison between Measured Spectra and Geant
Now, we take the average of all spectral measurements and compare to Geant4 simulations
Things to note:
Yttrium K strengths are comparable
Tungsten L lines not promiment in G4 Low Energy

11. Source to Source Variation
Here is the same data presented quantitatively as HVL…
Things to note:
Large src-src variation
G4 is situated right in the middle of the variation

12. Aging Effects
Because the lifetime of the src is 2 hours, this allows us to observe the effect of src aging…
Things to note:
HVL Changes are significant
No definitive trends

13. 2-D Dose Distribution Gafchromic EBT films Lucite Phantom
Src parallel to film
Src-film: 1.5 cm
Exposure time: 5 min
Max dose: 5 Gy
So that is it for beam quality, now we move on to distribution…
Calibration
Ir-192 calibrated in water
Film scanning
Document scanner
Image registration
Matlab correlation routine

14. Gaf EBT in Lucite: Relative Dose distribution at 1.5cm
Things to note:

15. Conclusion Geant4 Bremsstrahlung production comparable with EGS
Angular distribution
Bremsstrahlung mean free path
Bremsstrahlung spectrum
Spectral measurements
Spectra in agreement with Geant4
Attenuation curve measurements
HVL in agreement with spectral HVL and Geant4
Observed factors affecting beam quality
Source variation
Aging effect
Dose distribution measurements
General agreement with Geant4
Deviation in forward and aft directions

16. Acknowledgement This project is partially funded by Xoft, Inc.
CdTe spectrometer funded by Research Institute of MUHC