Are cancer risks from radiation probabilistic (stochastic) or deterministic (tissue reaction)?

Radiation risks for cancer are classified into probabilistic effects because the induction of oncogenic mutations in a single epithelial cell is thought to be causally related to the process of carcinogenesis. However, two observations remain unanswered by the theory. One is a parallel shift of mouse survival curves towards younger ages after a radiation exposure, and the other is a decreasing trend of relative risk for cancer with an increase in postexposure time. Both are readily explained if it is assumed only that a radiation exposure causes a shift of the spontaneous tumours to appear earlier as a result of tissue damage which favours a transformed cell(s) to grow faster. In line with this notion, there are reports showing that exposures to high doses of acute radiation did not increase the proportion of mice dying from tumours, while their lifespans were shortened. In contrast, although there are reports showing a dose-related increase in the proportion of tumour deaths, the studies were either prematurely terminated or the ages were adjusted, and they are not considered to support the tumour induction model of radiation. As it seems that radiation carcinogenesis is a type of tissue response, it should be more appropriately classified as a deterministic effect of radiation. It is added that this article deals mainly with the data obtained under high-dose and high–dose-rate exposure conditions.