Development of a Monte-Carlo model for a miniature 50 kV x-ray source for electronic brachytherapy

The relevance of the study is driven by the advancement of high-precision radiation therapy methods, particularly electronic brachytherapy, which utilizes miniature X-ray tubes as radiation sources. One such source is the Xoft Axxent (50 kV). However, existing dose calculation protocols (e.g., TG-43) are insufficiently adapted for electronic sources due to their specific spectral and geometric characteristics. This creates a need for the development of accurate and universal models to correctly calculate dose distributions in clinical practice. The aim of this work is to develop and verify a geometric model of the miniature Xoft Axxent X-ray source for electronic brachytherapy using the Monte Carlo method. The tasks included: constructing a geometric model of the source based on an analysis of published data, simulating the bremsstrahlung spectrum, calculating dosimetric parameters—the radial dose function and the anisotropy function—and comparing them with published reference data. The simulation was performed using the MC software code based on EGS4 physics. The results showed that the mean energy of the obtained spectrum was 26.72 keV, which is consistent with the manufacturer's data (deviation 0.46%). However, significant discrepancies in the radial dose function (up to 38.4%) were identified for the source configuration without an applicator, indicating limitations of the applied MC code for accurately describing dose distributions in the kilovolt range. The developed model serves as a basis for creating a universal virtual source for brachytherapy treatment planning systems.