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The RBE of the 85 MeV proton beam produced at the cyclotron of Louvain-la-Neuve using 60Co gamma rays as the reference radiation was determined for survival of Chinese hamster ovary cells in vitro and for intestinal crypt regeneration in mice in vivo. Cell survival curves determined at different depths yielded, for a surviving fraction (SF) of 0.01, RBE values of 1.11 +/- 0.05 at the initial plateau of the unmodulated beam, 1.10 +/- 0.03 at the middle of a 0.5-cm spread-out Bragg peak (SOBP), 1.03 +/- 0.03 at the beginning of a 3-cm SOBP and 1.07 +/- 0.03 at the end of a 3-cm SOBP. The highest RBE values were obtained at the middle of the 0.5-cm SOBP and at the end of the 3-cm SOBP (RBE = 1.22 and 1.16, respectively, at SF = 0.5), although the variations are not statistically significant. Irradiations with 3-Gy fractions separated by an interval of 3.5 h yielded RBEs of 1.11 +/- 0.30 and 0.90 +/- 0.32 at the initial plateau and at the middle of the 0.5-cm SOBP, respectively. Irradiations of mice at the middle of the 3-cm SOBP yielded an RBE of 1.08 +/- 0.03 for 20 regenerated crypts at a proton dose of 12.3 Gy.
Gueulette et al. (Mon,) studied this question.