RBE evaluation during the vertebral and brain cancer treatment using low-kV X-rays: A hybrid simulation approach

Intraoperative radiotherapy (IORT) using INTRABEAM machine and specific needle applicator is a candidate modality for vertebral and brain cancer treatment. Current study aims to evaluate the relative biologic effectiveness (RBE) values of low energy X-rays emitted from the surface of needle applicator during the IORT of vertebral and brain tumors, through a hybrid Monte Carlo (MC) simulation approach. To estimate RBE values, Geant4 was employed to simulate the INTRABEAM X-ray source along with needle applicator as well as scoring released secondary electron spectra within considered tissues. Then, RBE values were estimated by Monte Carlo damage simulation (MCDS) code. An increasing trend of RBE DSB (RBE for DNA double strand breaks) values was found by increasing the distance from applicator surface. The maximum RBE DSB values were obtained for vertebral bone (C4) among the investigated tissues due to higher physical density and Z eff values. Cell oxygenation level also affected RBE DSB with a maximum value of 64 % when moving from 0.3 % to 4 % level. From the results, it can be concluded that RBE values during the low energy X-ray IORT of brain and vertebral column tumors using dedicated needle applicator can be affected by considerable differences in tissue composition and cell oxygenation level.