The biophysical stage of radiation carcinogenesis

The dependence of the induction of cancer on the absorbed dose of ionizing radiations has been specified in terms of increasing complexity. The first notion of the linear hypothesis is now frequently replaced with a dependence on both the first and second powers of the dose the linear-quadratic model, which implies proportionality at low doses only. Microdosimetric considerations and in particular the theory of dual radiation action would be in accord with this relation if tumors were to arise from single cells as the result of a transformation that depends only on the radiation received by the cell. In this case it must be expected that the linear portion of the dose-effect curve is dose rate independent but that the quadratic component may decrease with decreasing dose rate because of repair. However it was shown some time ago that the dose-incidence relation of a neoplasm indicates a non-autonomous response because of departure from a linear dependence when the mean number of events in cells is much less than one in neutron irradiations. Another discrepancy is the repeated observation that reduction of dose rate, while resulting in the expected lessening of the effectiveness of low-LET radiation, increases the effectiveness of neutrons especially in the case of oncogenic cell transformation. 32 refs., 3 figs