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Characterization of Relative Biological Effectiveness (RBE) for Proton Therapy in Human Cancer Cell Lines

M Herman

M Howard , M Herman*, S Anderson , H Wan Chan Tseung , J Sarkaria , C Beltran , Mayo Clinic, Rochester, MN


SU-I-GPD-T-108 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Relative biological effectiveness (RBE) is utilized to account for the differences in biological effect from different radiation types. The RBE for proton therapy remains uncertain as it has been shown to vary from the clinically used value of 1.1.

Methods: Three cell lines were irradiated (CHO, Chinese hamster ovary; A549, human lung adenocarcinoma; and T98, human glioma) and assessed for cell survival using clonogenic assay. Cells were irradiated with 71 and 160 MeV protons at depths along the Bragg curve and 6 MV photons to various doses. The dose averaged lineal energy (yD), was measured under similar conditions as the cells using a microdosimeter. Dose averaged LET (LETd) was also calculated using Monte Carlo (MC) simulations. Survival data were fit using the linear quadratic model. RBE values were calculated by comparing the physical dose (D6MV/Dp) that results in 50% (RBE_0.5), 10% (RBE_0.1) cell survival, and survival after 2Gy (RBE_2Gy).

Results: For 10% and 50% survival, the RBE for all three cell lines increased with decreasing proton energy (or increased depth) and was higher at 50% survival compared with 10% survival. The RBE at 2Gy also increased with a decrease in proton energy in all cases, within experimental error. An increase in RBE corresponds with an increase in measured yD and calculated LETd.

Conclusion: Cellular response to radiation is varied and can be seen in the data from CHO vs. A549 and T98 cell lines irradiated with 71 MeV protons. Both A549 and T98 cells generally had higher RBE values, indicating a greater biological response to protons. The RBE values in this study vary from 0.89– 2.40, indicating the clinical value of 1.1 may not be suitable in all cases.

Funding Support, Disclosures, and Conflict of Interest: NIH grant T32-HL105355 PI: Gary Sieck Pre-doctoral Trainee (M Howard)

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