Toward Patient-Specific Monte Carlo Dosimetry of the Vektra Electronic Brachytherapy System: A Proof of Concept
Abstract
Purpose
This work demonstrated the feasibility of performing in-water dose calculations with Monte Carlo (MC) simulations of the Vektra Electronic Brachytherapy (EB) system (Empyrean, Boca Raton, FL). The Vektra is a novel method of EB, using beam steering and internal shielding to produce an anisotropic dose.
Methods
CAD files and material composition data were used to model the Vektra source within a Geant4 usercode. Incident electron beams were simulated, and interactions within the target produced bremsstrahlung and characteristic x-rays. To improve efficiency, bremsstrahlung splitting was implemented in the target region with a splitting number N = 100. The energy, position, and momentum of emerging x-ray photons were scored at the device tip, and stored as phase-space files. Phase-spaces were generated for each tube potential (50, 60, 70,80, 90, 100 keV). The phase space for the 50 kV beam was imported into RapidBrachyMCTPS, an MC-based treatment planning system, to calculate the dose to a voxelized water phantom.
Results
Phase space simulations for all beams exhibited bremsstrahlung spectra and characteristic x-rays of molybdenum. The characteristic x-rays for tungsten are observed for electron beams above 60 kV. For electron beam energies of 50, 60, 70,80, 90, 100 keV the respective mean photon beam energies are 27.7, 31.6, 35.5, 38.8 41.9 and 44.7 keV. The relative dose calculations for 50 kV demonstrate the anisotropic distribution of the x-rays. Dose deposited on the contralateral side of the Vektra was found to be 13.7% +/- 0.4% that of the dose on the ipsilateral side, treating the tumour.
Conclusion
This work successfully developed an MC model of the Vektra EB system. Dose-to-water calculations will be performed for all available beam energies, and experimentally validated. Future patient-specific dose calculations in RapidBrachyMCTPS will form the foundation for incorporating the MC dosimetry model into the Vektra treatment planning system