Experimental studies on neutron carcinogenesis: their possible relevance for radiation protection

With respect to radiation protection the following aspects of experimental studies on neutron carcinogenesis are of relevance: Assessment of the nature of dose-response relationships; Determination of the relative biological effectiveness of radiations of different quality; Effects of fractionation or protraction of the dose on tumour development. A number of studies on in vitro carcinogenesis and on tumour induction in animals after irradiation with neutrons and X-rays are summarized. For the analysis of tumour data in animals, specific approaches, such as nonparametric actuarial methods and proportional hazard functions have to be applied. The dose response curves for radiation induced cancers in different tissues vary in shape. This is exemplified by studies on myeloid leukemia in mice and mammary tumours in different rat strains. The results on radiation carcinogenesis in animal models clearly indicate that the highest RBE values are observed for neutrons with energies between 0.5 and 1 MeV. On the basis of such results it might be concluded that the maximum quality factor of 10 for neutrons should be increased. Based on current evidence, an increase by a factor of 2 to 3 seems more realistic than a tenfold rise. The experimental studies performed up to the present do not allow to quote organ specific quality factors. The diversity of dose-response relationships point to different mechanisms involved in the induction of different tumours in various species and even in different strains of the same species