Dosimetric evaluation of tissue heterogeneity for electronic brachytherapy (EBT) source in high dose rate gynecological (GYN) irradiation

Abstract

The purpose of this work is to quantitatively evaluate the dosimetric impact of differing materials in both homogenous and heterogeneous settings for an electronic brachytherapy source. Monte Carlo Simulations were created using the GEANT4 toolkit (version 10.0). The XOFT AxxentTM EBT source was modeled as a point x-ray source placed inside of a titanium applicator. The geometry in this simulation includes a titanium applicator with a length of 107 mm, the applicator thickness for the side walls and cap is 0.4 mm and 0.5 mm, respectively. The applicator is placed in a volume where the materials are varied between numerous ICRU standard materials such as: water, skeletal muscle, cortical bone and adipose tissue. Two dwell positions were simulated at 0 mm and 12 mm from the tip of the applicator where different TG-43 parameters were provided to account for geometry effect of the applicator. The energy spectrum for the 50 kVp XOFT x-ray source was provided via the manufacturer when the physical parameters of the source were characterized. The dose distributions were recorded using a parallel three dimensional mesh with a size of 30 × 30 × 30 cm3 with 1 × 1 × 1 mm3 voxels placed at the source origin. For all simulations, the low-energy Livermore physics list was implanted with Auger Electron, particle induced x-ray emission and x-ray fluorescence options enabled. The range cuts for electrons and photons were set at 0.01 mm. The radial dose functions for the XOFT source with/without applicator were also measured in a water phantom using ion chambers and Gafchromic EBT3 film. The simulated dose absorbed along the transverse axis was normalized at the distance of 1 cm and then compared with the measurement results as well as the calculations from Varian BrachyVision treatment planning system using standard TG-43 formalism. The simulated dose profile in water was within 7.55% of measured and calculated TG-43 results for the distance within 5 cm. However, a greater difference in dose comparison was shown when different materials were used especially for a high density bone material. For a heterogeneous geometry, differences in the absorbed dose varied by a factor of 5 when compared to the TG-43 dose formalism. Significant discrepancies in dose distributions have been shown when different tissue types were used in contrast with the water only assumption for TG-43 formalism. Simple adaption of TG-43 formalism could lead to huge errors in patient dose calculation especially for the low energy EBT source.