Measurements of relative biological effectiveness (RBE) have been made on the range-modulated 70 MeV proton beam at TRI-UMF using a precise cell sorting survival assay. In this study, Chinese hamster V79-WNRE cells were suspended in medium containing liquid gelatin at 37°C in irradiation tubes and the gel was allowed to solidify by cooling to 4°C. Complete cell survival responses were measured at 11 positions with 2 mm spacing within a proton stopping peak width of approximately 2 cm. Survival responses after proton irradiation were compared with responses to60 Co γ rays measured at the same time, and RBE values were determined as a function of both dose and depth. Above doses of 4 Gy, the average RBE for these cells throughout the modulated proton stopping distribution was 1.21 ± 0.05, measured at a survival of 1%. However, we also observed that, within the spread-out Bragg peak, the RBE increased with increasing depth, from ∼1.2 at the proximal part to >1.3 at the distal part of the peak. At the distal edge of the stopping distribution, the RBE value increased significantly, to an extent that may be of concern when this region of the treatment volume is close to sensitive tissues. Below 4 Gy, the RBE value was also dependent on radiation dose, increasing significantly to values of approximately 1.37 and 1.56 at 2 and 1 Gy, respectively. Our results illustrate that the use of a single RBE value in different irradiation protocols can be an oversimplification, and argues for the use of "proton gray doses" rather than "γ-ray equivalent grays."

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