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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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