Biological bases for high RBE values for late effects of neutron irradiation

The late effects of fractionated irradiation with neutrons have been relatively more severe than after x-irradiation. Reasons for the RBEn/x being higher for late than for acute effects may include: (1) Late effects are reduced more by fractionation of X ray doses than are acute effects, whereas, with neutrons, the fractionation response is the same in rapidly-and slowly-responding tissues; (2) Late-responding tissues are less "sensitized" (and are, therefore, relatively protected) by redistribution throughout the division cycle during a fractionated regimen than are acutely-responding tissues: since neutron responses are less affected by cell-cycle distribution than are X ray responses, the relative protection of slowly-responding tissues is less; (3) The target cells for late, but not acute injury, may repair potentially lethal damage after X ray, but not after neutron exposure. Thus, the dissociation of RBE values for acute and late injury reflects mainly the dissociation between acute and late responses to conventional fractionated X ray regimens and, from the point of view of complications of radiotherapy, we should not condemn neutrons but praise X rays. Since fractionation of neutron doses into increments equivalent to those used in X ray therapy does not provide a preferential sparing of slowly-responding tissues, it is reasonable to shorten the overall duration of neutron treatment to deliver the total dose tolerated by the relevant "late-effects" tissue(s) in the shortest time consistent with acceptable acute responses.