Aspects of neutron therapy based on an analysis of relationships between RBE and dose

The term “relative biological effectiveness” (RBE) is in almost universal use for comparing effects of radiations of different quality. The accepted definition (Storer et al., 1957), “Inverse of the ratio of doses to give the same effect” might, perhaps, suggest that it is a single valued parameter. However, it is now generally realized that in the comparison of radiations of different quality given as single doses the RBE usually depends on the level of effect under consideration and the state of oxygenation of the irradiated material; also, in some instances, on the dose-rate. When a fractionated scheme is used, the size and number of dose fractions also affect the RBE. It has been pointed out that for a given system the RBE depends on the dose per fraction when all fractions in a treatment are of equal size (Fowler and Morgan, 1963; Field, 1969). This is true, of course, only if the effect of any fraction does not in itself change the nature of the response to a subsequent one. Comparing observations made on various animal tissues after irradiation in vivo by cyclotron neutrons and 250kV X rays, Field showed that many of these conformed to the same pattern of reduction in RBE as the dose of neutrons administered per fraction was increased. Hornsey (1970a), using Field's method of plotting the results, pointed out that, despite the rough similarity, different tissues varied significantly in the dependence of RBE on neutron dose.