RBE determinations have been performed in all neutron therapy centres prior to any clinical application. In some centres, the minimum checks were done in order to start the treatments in safe conditions. In other centres, extensive investigations were performed comparing several biological systems. A large amount of data was thus accumulated. However, only a few investigations were performed in order to systematically assess the RBE variation as a function of neutron energy for the same biological system, which could allow an accurate exchange of information between centres. From the available radiobiological data, it can be concluded that RBE increases with decreasing neutron energy, but the slope of the RBE/neutron energy relationship depends on the biological system and endpoint. RBE values as high as 1.53 were observed for d(20) + Be relative to p(65) + Be neutrons. Even when comparing the modem neutron therapy facilities (neutrons produced by protons with energy higher than 40 MeV), significant RBE variations are still observed, which needs to be taken into account when designing multicentre therapeutic protocols.

The RBE of p(75) + Be neutrons relative to d(50) + Be neutrons has been determined for chromosome aberrations induced in Allium cepa (onion) roots. Two biological criteria were selected: the average number of aberrations (mainly fragments) per cell in anaphase and telophase, and the percentage of aberration-free cells. The influence of sampling time (3 to 7 h incubation) between irradiation and fixation was investigated systematically. This factor did not significantly influence the results. The RBE values of p(75) + Be neutrons compared to those of d(50) + Be neutrons were 0.85 (0.79-0.91) and 0.87 (0.80-0.95) for the first and the second criteria, respectively. In previous experiments for the same beams, we found an RBE of 0.90 (0.86-0.94) for survival of V79 cells (D0 ratio), 0.96 (0.93-0.99) for the intestinal crypt cell system, and 0.83 (0.70-0.96) for Vicia faba growth delay.

We studied in vitro the influence of ionizing radiations on the life span of non-transformed HF 19 human fibroblasts. The life span of surviving clones was found to be reduced when the cells had received two or three doses of 6 Gy separated by an interval of 15 doublings. In addition, this reduction in life span was greater when the cells were older at the time of irradiation.

RBE/absorbed dose relationship of d(50)-Be neutrons was determined for the induction of chromosome aberrations in Allium cepa onion roots. Neutrons are produced at the cyclotron "Cyclone" by bombarding a thick beryllium target with 50 MeV deuterons. Two biological criteria were selected: (1) mean number of aberrations (mainly breaks) per cell in anaphase and telophase, (2) fraction of intact cells in anaphase and telophase. For the two criteria, RBE increases continuously from about 7 to 12 as the neutron absorbed dose decreases from 0.4 to 0.1 Gy. RBE values for the first criterion are slightly higher than for the second one. This observation is interpreted in terms of the analysis of the distribution of the aberrations in the cells. In logarithmic coordinates, RBE/absorbed dose relationships for the two criteria are almost linear with a slope close to -1/2. RBE values observed for induction of chromosome aberrations in Allium cepa are higher than those generally observed for biological effects related to mammalian cell lethality.

The relative biological effectiveness (RBE) and the oxygen enhancement ratio (OER) of 50 MeV neutrons have been investigated with three mammalian cell lines in culture. With EMT6 cells and the fibroblastic cell line HF 19, we have obtained RBE values of 2.9 and 1.9 respectively at a survival level of 5 x 10−1. The OER of EMT6 and V 79 cells were 1.55 and 1.90 respectively which corresponds to a gain factor of 1.70 and 1.95. These values are comparable to those found in the literature. The biological interpretation of some observed late scleroses in clinical practice is discussed in the light of the in vitro radiosensitivity of HF 19 fibroblast.