Dosimetric characterization of the Xoft Axxent electronic brachytherapy source: A Monte Carlo study

The Xoft Axxent electronic brachytherapy (eBT) source is a miniature x-ray tube operating at 40-50 kVp. While a dosimetric formalism specific to eBT has been developed, this work focuses on determining TG-43 parameters using that framework.

To evaluate the accuracy of the TOPAS Monte Carlo user code in simulating dosimetric parameters for Xoft eBT sources.

TOPAS was used to simulate electron transport through a cylindrical tungsten target to assess absorbed, backscattered, and transmitted electron fractions, along with bremsstrahlung production. Photon spectra were simulated on the surface of both bare and applicator-inserted sources (S7500 and S7600 models) using TOPAS and benchmarked against EGSnrc simulations. Dose to water was scored to derive radial dose function and polar anisotropy. Air kerma at 50 cm radial distance was used to calculate the dose rate conversion coefficient. Simulated results were compared against EGSnrc and available experimental data.

TOPAS and EGSnrc simulations showed close agreement in electron absorption (max difference 0.47%), backscatter (0.63%), and transmission (0.21%). Average bremsstrahlung energy differed by only 0.09%. Spectral average energy differences between codes were 0.3% for the bare source and 1% for the applicator case. Radial dose functions agreed within 3% for both source types, and anisotropy functions also matched well. Slightly larger discrepancies were noted for the bare source than for the source in applicator.

TOPAS demonstrated strong agreement with EGSnrc and measured data, with marginally improved accuracy-potentially due to more realistic modeling of manufacturing-related spectral differences.