RBE variation as a function of depth in the 200-MeV proton beam produced at the National Accelerator Centre in Faure (South Africa).

Abstract

Thorough knowledge of the RBE of clinical proton beams is indispensable for exploiting their full ballistic advantage. Therefore, the RBE of the 200-MeV clinical proton beam produced at the National Accelerator Centre of Faure (South Africa) was measured at different critical points of the depth-dose distribution. RBEs were determined at the initial plateau of the unmodulated and modulated beam (depth in Perspex = 43.5 mm), and at the beginning, middle and end of a 7-cm spread-out Bragg peak (SOBP) (depths in Perspex = 144.5, 165.5 and 191.5 mm, respectively). The biological system was the regeneration of intestinal crypts in mice after irradiation with a single fraction. Using 60Co gamma-rays as the reference, the RBE values (for a gamma-dose of 14.38 Gy corresponding to 10 regenerated crypts) were found equal to 1.16 +/- 0.04, 1.10 +/- 0.03, 1.18 +/- 0.04, 1.12 +/- 0.03 and 1.23 +/- 0.03, respectively. At all depths, RBEs were found to increase slightly (about 4%) with decreasing dose, in the investigated dose range (12-17 Gy). No significant RBE variation with depth was observed, although RBEs in the SOBP were found to average a higher value (1.18 +/- 0.06) than in the entrance plateau (1.13 +/- 0.04). An RBE value slightly larger than the current value of 1.10 should be adopted for clinical application with a 200-MeV proton beam.