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Spatial and Energy Variations in Relative Biological Effectiveness (RBE) for DNA Double Strand Break (DSB) Induction of Spot Scanning Proton Beams

V Moskvin

V Moskvin1*, R Stewart2 , J Farr1 , (1) St. Jude Children's Hospital, Memphis, TN, (2) University of Washington, Seattle, WA.


TU-H-CAMPUS-TT-4 (Tuesday, August 1, 2017) 4:30 PM - 5:30 PM Room: Therapy ePoster Theater

Purpose: To underline the need to incorporate spatial variations in proton RBE into proton therapy treatment planning. In this study, we present findings on spatial and energy variations in the relative biological effectiveness (RBE) of spot scanning proton therapy beams.

Methods: Particle- and energy-specific information on an endpoint of DNA double strand break (DSB) induction from the Monte Carlo Damage Simulation (MCDS) was integrated into the FLUKA code systems for protons and other light ions. The depth and lateral spatial RBE for DSB induction of protons beams were computed in therapeutic interval of energies from 69.4 MeV to 221.3 MeV for anoxic conditions.

Results: We found that the proton beam RBE at the area the Bragg peak (BP) and distal edge has strong dependence on initial beam energy. RBE is close to 1.25 for high energies 160-221 MeV, 1.35 at 100 MeV and increases to 1.49 at 69.4 MeV. Lowering the beam range to the skin with the range shifter will increase RBE further, thus the RBE for 40 MeV reaches 1.73. The RBE is independent on the initial size of the spot in the interval from 1 mm to 15 mm. The RBE increases laterally to the beam axis both at the BP and plateau areas at distances larger than the level of 10% dose in lateral profiles. The lateral and distal to BP increase of the RBE is correlated with the dose average proton energies, and associated with increases of protons fraction from sequential (n,p) + (p,n) reactions with increasing depth beyond the BP and laterally.

Conclusion: The dose fields may show higher efficacy for treatment of small tumors at shallow depths. The lateral spatial increase of RBE under certain conditions may need additional consideration between critical avoidance organs and beam spot placements.

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