Purpose: This study aims at providing relative biological effectiveness (RBE) data under reference conditions accounting for the determination of the "clinical RBE" of protons.
Methods and materials: RBE (ref. (60)Co gamma-rays) of the 200 MeV clinical proton beam produced at the National Accelerator Centre (South Africa) was determined for lung tolerance assessed by survival after selective irradiation of the thorax in mice. Irradiations were performed in 1, 3, or 10 fractions separated by 12 h. Proton irradiations were performed at the middle of a 7-cm spread out Bragg peak (SOBP). Control gamma irradiations were randomized with proton irradiations and performed simultaneously. A total of 1008 mice was used, of which 96 were assessed for histopathology.
Results: RBEs derived from LD50 ratios were found not to vary significantly with fractionation (corresponding dose range, approximately 2-20 Gy). They, however, tend to increase with time and reach (mean of the RBEs for 1, 3 and 10 fractions) 1.00, 1.08, 1.14, and 1.25 for LD50 at 180, 210, 240, and 270 days, respectively (confidence interval approximately 20%). alpha/beta ratios for protons and gamma are very similar and average 2.3 (0.6-4.8) for the different endpoints. Additional irradiations in 10 fractions at the end of the SOBP were found slightly more effective ( approximately 6%) than at the middle of the SOBP. A control experiment for intestinal crypt regeneration in mice was randomized with the lung experiment and yielded an RBE of 1.14 +/- 0.03, i.e., the same value as obtained previously, which vouches for the reliability of the experimental procedure.
Conclusion: There is no need to raise the clinical RBE of protons in consideration of the late tolerance of healthy tissues in the extent that RBE for lung tolerance was found not to vary with fractionation nor to differ significantly from those of the majority of early- and late-responding tissues.