SU‐E‐T‐571: Microdosimetric Characterization of Proton Biological Effectiveness

Purpose: The knowledge of cell repair kinetics is critical in order to understand relevant aspects of proton therapy such optimal fractionation. High LET radiation induces larger proportions of more complex DSBs that might not be resolved in standard interfractional times (24h). We propose a method to characterize the cell repair kinetic at different positions within the spread‐out Bragg peak (i.e. as a function of proton LETd) and use this information to model normal tissue complication probabilities (NTCP) as a function of LETd Methods: Repair kinetics curves will be obtained using U251‐glioblastoma cells plating over microscope slides that are immersed in culture‐medium that will be exposed along the axis of a proton‐beam (10cm in range, 2cm modulation) of 1Gy. Culture is immersed within a solid‐water block to place the distal edge of the proton‐beam within the slide. gamma‐H2AX foci along the slide in the beam axis is imaged with a microscope at different positions in order to correlate number of gamma‐H2AX foci with dose and LET, where dose is obtained from a strip of gafchromic film placed on the side of the slide. Each slide is fixed at different time points. Results: We will present the correlation between an increase of LET and the increase of the gamma‐H2AX along the depth within the proton beam, at different post‐irradiation times and doses. Different fractions of unrepaired damage at 24h after irradiation will be presented at each depth of the proton beam, with larger fractions at the distal edge of the beam, where normal tissue exist. Larger values of NTCP are therefore observed at the distal edge. Conclusion: Slower repair kinetics is observed at the distal edge of a proton beam, and this study shows a method to obtain such information and the correlation between increased LET and increased NTCP. This research is supported by the Department of Radiation Oncology at the Hospital of the University of Pennsylvania as a pilot project.