RBE VARIATION AS A FUNCTION OF DEPTH IN THE 200-MEV PROTON-BEAM PRODUCED AT THE NATIONAL-ACCELERATOR-CENTER IN FAURE (SOUTH-AFRICA)

Background and purpose: Thorough knowledge of the RBE of clinical prot on beams is indispensable for exploiting their full ballistic advantag e. Therefore, the RBE of the 200-MeV clinical proton beam produced at the National Accelerator Centre of Faure (South Africa) was measured a t different critical points of the depth-dose distribution. Material a nd methods: RBEs were determined at the initial plateau of the unmodul ated and modulated beam (depth in Perspex = 43.5 mm), and at the begin ning, 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 sy stem was the regeneration of intestinal crypts in mice after irradiati on with a single fraction. Results: Using Co-60 gamma-rays as the refe rence, the RBE values (for a gamma-dose of 14.38 Gy corresponding to 1 0 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 decreasi ng dose, in the investigated dose range (12-17 Gy). No significant RBE variation with depth was observed, although RBEs in the SOBP were fou nd to average a higher value (1.18 +/- 0.06) than in the entrance plat eau (1.13 +/- 0.04). Conclusion: An RBE value slightly larger than the current value of 1.10 should be adopted for clinical application with a 200-MeV proton beam. (C) 1997 Elsevier Science Ireland Ltd.