Monte Carlo based analysis and evaluation of energy spectrum for low-kV IORT spherical applicators

Low-kV IORT is an increasing modality for breast cancer treatment. Soft X-rays from INTRABEAM (Carl Zeiss Meditec AG, Oberkochen, Germany), a dedicated IORT device along with special spherical applicators are employed for this purpose. A Monte Carlo model of INTRAMBEAM and spherical applicators are introduced in the current study to evaluate the dosimetric and physical characteristics of emitted X-rays from the bare probe and different applicator diameters. X-ray probe and different applicator diameters of 1.5cm to 5cm were simulated by GEANT4 Monte Carlo Toolkit. Then, the validity of the simulated model was evaluated by comparing the Monte Carlo based PDD (percentage depth dose) and anisotropy data with those reported by the manufacturer. Finally, the physical characteristics of X-rays such as the mean and most probable energy as well as the LET of secondary electrons were obtained and analyzed. There was a good agreement between the simulated and reported PDDs for bare probe and different applicator diameters. The anisotropy values were also within an exemplary range reported by the manufacturer. The X-ray mean energy shifts from 25.6keV to 28.6keV with variations of applicator diameter. The maximum variation of the secondary electron LET was 9% with changing the applicator diameter. The usefulness of GEANT4 Toolkit for Monte Carlo based commissioning of INTRABEAM machine was confirmed in the current study. The variations of the applicator diameter slightly changed the physical characteristics of low-kV X-rays and LET of secondary electrons which cannot considerably affect the relative biologic effectiveness (RBE) for different applicator diameters.