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

The RBE of protons has been investigated on the external beam of the proton accelerator, specially adjusted to clinical use. 137 Cs was used as a standard for comparison. The mean flux of particles in the proton beam was 2-10 8 protons cm 2 in each 1 -6 /xs 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. There has recently been considerable interest in the use of accelerated protons for radiotherapy (Lawrence, 1967; Larsson, 1967). For this reason it is important to determine the relative biological efficiency (RBE) of these charged particles. The RBE of proton beams has been investigated thoroughly in the plateau region of the Bragg curve (Jarmonenko and Konoplannikov, 1965). The RBE in the peak region has been determined in a limited number of experiments (Larsson and Kihlman, 1960; Sweet et al., 1967). We have used HeLa and Chinese hamster cells to measure the RBE of the proton beam from the synchrotron which has been specially adapted to medical use. MATERIALS AND METHODS Cell culture technique HeLa cells and Chinese hamster cell line B-lld-ii-FAF-28, clone number 431, were grown in monolayer culture and cell survival was estimated by their clone-forming ability. Monodispersed cells were explanted in glass Carrel flasks of 6 cm diameter containing 6 ml. of medium. The flasks were incubated at 37°C with 5 per cent CO2 in air for three hours, and then rubber stoppers were substituted for cotton plugs. This procedure helped to keep the pH of the medium at a constant level during transportation and irradiation of the cells. The irradiation was performed four to seven hours after explanting. During this period the radiosensitivity of the cells was found to be constant, in good agreement with other findings (Barendsen and Walter, 1964). In the experiments in which cells were irradiated in the Bragg peak region they were kept in Carrel flasks with a thin Mylar film bottom. For the analysis of chromosomal aberrations Chinese hamster cells were grown on thin Mylar sheets of the dimensions of smaller Carrel flasks. Seven to eight hours after irradiation the cells were fixed and stained by aceto-orcein. Cells with bridges and acentric fragments were countered during late