The RBE of accelerated protons in different parts of the Bragg curve

Abstract

The RBE of protons has been investigated on the external beam of the proton accelerator, specially adjusted to clinical use. 137Cs was used as a standard for comparison. The mean flux of particles in the proton beam was 2·108 protons cm−2 in each 1·6 μs pulse, delivering a dose of about 20 rads in the plateau region of the curve and 80 rads at the Bragg peak, the relative height of the latter being 4. The RBE of 90 MeV protons ranged from 0·75 for the survival of HeLa cells to 0·68 for the survival of Chinese hamster cells. The RBE determined by the abnormal anaphase scoring technique showed a value of 0·77 for hamster cells. The biological effectiveness of proton irradiation in the Bragg peak (mean energy 8 MeV) in comparison to the plateau region was found to be l·44±0·08 as estimated on the basis of clone formation and 1·85±0·15 on the basis of the induction of chromosome aberrations. Some data concerning the absorbed dose determination for cells irradiated at the boundary between the glass and the nutrient media are discussed.