Author: Anthony Hong Cheol Lim, Alexander R ODell, Alexander Stanforth, Chris C. Wang 👨🔬
Affiliation: Georgia Institute of Technology, Emory University 🌍
Purpose: The purpose of this work is to investigate the variation of proton RBE, with respect to depth, in the context of clinical SOBP. Employing a novel approach, this study conducts a Monte Carlo simulation of DNA damage at a SOBP ‘spot’ on the cell-by-cell basis. Subsequent analysis of the DNA damage for each cell through MEDRAS enables the generation of survival curves for each depth, allowing for RBE assessment. This methodology provides a framework for assessing the RBE of proton beams with considerable detail, balancing accuracy with practicality.
Methods: A SOBP ‘spot’ was simulated in TOPAS, with the source at 30cm SSD from a water tank’s surface. Cylindrical slabs were placed at several depths and the kinetic energies of primary protons and delta rays, passing through these slabs were tallied, yielding the primary proton and delta ray energy spectra at each depth. These spectra are then used to generate SDD (standard DNA damage format) files, describing DNA double strand breaks (DSBs) for individual cells post-irradiation, by fetching data from the pre-calculated SDD libraries using TOPAS-nBio. The SDD files are then analyzed using MEDRAS to determine the biological outcomes on a cell-by-cell basis for several different doses and for each considered depth.
Results: The results of initial DSBs/Gy/GBP’s at each depth align with experimental data. The results of cell survival indicate an increase in RBE towards the SOBP’s distal end, which is also consistent with the previously published results. The cell survival values predicted by MEDRAS are consistently lower than experimental data.
Conclusion: The results indicate an increase in RBE of cell survival towards the SOBP’s distal end, which is consistent with the previously published results. The inconsistency of predicted cell survival with experimental data, suggests that a more refined model in simulating DNA repair is necessary for improved accuracy.