SU‐E‐T‐495: Monte Carlo Simulations of Relative DNA Damage From KV CBCT Radiation

Purpose: Daily imaging protocols using kilovoltage cone beam CT (kV CBCT) have the potential to accumulate non‐trivial doses (up to several percent of the prescription dose) to the imaged volume which could have implications for planning decisions or the induction of secondary cancers. When CBCT dose is added to therapeutic dose, it may be necessary to account for differences in the relative biological effectiveness (RBE). We conducted a Monte Carlo study to investigate the relative number and complexity of DNA double strand breaks (DSBs) that Result from kV CBCT radiation compared to Co‐60 as a reference for general external beam radiation. Methods: A method was developed for coupling the radiation transport code (PENELOPE) with a Monte Carlo Damage Simulation (MCDS) code that predicts relative numbers and complexity of DSBs. The PENELOPE simulations generated electron spectra relevant for the MCDS input consisting of (i) electrons generated outside of a cell nucleus that drift in and (ii) electrons created within the nuclear volume, both without the continued slowing within the nucleus itself that is simulated implicitly by the MCDS code. Various configurations were considered (surface/depth, bowtie filter thickness, kV energy, and degrees of oxygenation). Results: The RBE for DSB induction in the kV CBCT sources ranged from 1.14 to 1.19 with increased complexity compared to that of Co‐60 for the majority of conditions; however, values as high as 1.45 were calculated in extreme circumstances. The RBE increased for anoxic conditions, at the phantom surface, for lower energies and for less filtration. Conclusions:Our results suggest kV CBCT sources produce more DNA damage than therapeutic radiation for a given absorbed dose and the relative difference can depend on several factors. This has potential implications for calculating the probability for inducing secondary cancers through the use of image guidance radiation.