Theoretical substantiation of biological efficacy enhancement for β-delayed particle decay 9C beam: A Monte Carlo study in combination with analysis with the local effect model approach

Purpose: To improve the efficacy of heavy ion therapy, β-delayed particle decay 9C beam as a double irradiation source for cancer therapy has been proposed. The authors' previous experiment showed that relative biological effectiveness (RBE) values at the depths around the Bragg peak of a 9C beam were enhanced and compared to its stable counterpart 12C beam. The purpose of this study was to explore the nature of the biological efficacy enhancement theoretically. Methods: A Monte Carlo simulation study was conducted in this study. First a simplified cell model was established so as to form a tumor tissue. Subsequently, the tumor tissue was imported into the Monte Carlo simulation software package gate and then the tumor cells were virtually irradiated with comparable 9C and 12C beams, respectively, in the simulations. The transportation and particle deposition data of the 9C and 12C beams, derived from the gate simulations, were analyzed with the authors' local effect model implementation so as to deduce cell survival fractions. Results: The particles emitted from the decay process of deposited 9C particles around a cell nucleus increased the dose delivered to the nucleus and elicited clustered damages around the secondary particles' trajectories. Therefore, compared to the 12C beam, the RBE value of the 9C beam increased at the depths around their Bragg peaks. Conclusions: Collectively, the increased local doses and clustered damages due to the decayed particles emitted from deposited 9C particles led to the RBE enhancement in contrast with the 12C beam. Thus, the enhanced RBE effect of a 9C beam for a simplified tumor model was shown theoretically in this study.