M Xing¹, Y Lai², Y Chi³, X Jia⁴*, (1) UT Southwestern Medical Center, Dallas, Texas, (2) University of Texas at Arlington, Arlington, TX, (3) University of Texas at Arlington, Arlington, TX, (4) The University of Texas Southwestern Medical Ctr, Garland, TX

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  Y Chi

TH-D-202-3 (Thursday, 7/14/2022) 11:00 AM - 12:00 PM [Eastern Time (GMT-4)]

Room 202

Purpose: Studying Relative Biological Effectiveness (RBE) of proton irradiation is important for the understanding of its biological effects. This study develops a mechanistic model to compute RBE based on microscopic Monte Carlo (MC) simulations of DNA damages.

Methods: We used our in-house developed GPU-based microscopic MC simulation tool, gMicroMC, to compute double strand breaks (DSBs) of a multi-scale DNA model caused by proton or x-ray irradiations. Because DNA DSB is the major factor affecting cell death, we assumed that there is a universal function of cell survival fraction (SF) on number of DSBs, which holds for both x-ray and proton irradiations. We first computed number of DSB as a function of dose under the reference 60Co irradiation and used published 60Co SF data to derive the universal function of SF on number of DSBs. We then considered a 155-MeV proton beam normally impinging to water. At each depth, we computed dose as a function of DSBs, and used the universal function between SF and number of DSBs to compute SF as a function of proton beam dose. Finally, RBE was computed as the ratio of 60Co and proton doses to reach the same endpoint of 10% SF. We also computed linear energy transfer (LET) at different depths.

Results: Proton RBE increases with depth from ~1.03 at the entrance region (~25.0mm), to 1.18 before the pristine Bragg peak (155.0 mm), ~1.32 at the Bragg peak (161.2 mm), and ~ 1.76 at the beam range (167.7 mm). Dose averaged LET at these depths were 1.9 keV/µm, 7.7 keV/ µm, 12.7 keV/µm, and 20.6 keV/µm, respectively.

Conclusion: The proposed mechanistic model can compute proton beam RBE. Calculation results generally agreed with experimental results published in previous studies.

Funding Support, Disclosures, and Conflict of Interest: Cancer prevention and research institute of Texas # RP160661

Keywords

Protons, RBE, Monte Carlo

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – computational dosimetry-Monte Carlo

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