Abstract
Cancer risks from radiation can be observed as an increase in mortality when compared to a control group. However, it is unknown if this increased risk results from the induction of cancer or from an earlier onset of cancer. In mouse studies, it has been repeatedly shown that after an irradiation, the survival curve is shifted toward lower ages, but remains parallel to the control curve, and the extent of the shift in time to lower ages is dose-dependent. This shift is not satisfactorily explained by the induction model which assumes that cancers in the exposed group consist of spontaneous and induced events. Consequently, it seems that this shift could be interpreted to mean that all animals in the exposed group had suffered from life shortening. Under this scenario, however, it turns out that the radiation effects can no longer be interpreted as the result of oncogenic mutations, because these effects would have to involve all tumors, and the effectiveness of radiation changes with the dose. This leads to the speculation that radiation exposures induce a broad range of tissue injuries, and that these injuries are subsequently subjected to longlasting systemic recovery processes which act as promoters for tumor cells. In other words, potential cancer stem cells which were located in the irradiated field can escape oncogenic damage but undergo stimulation later in life toward the development of malignancy from radiation-induced activated microenvironment. This is an unusual form of the non-targeted or bystander effects of radiation. It is worth noting that this model suggests that there could be a path or paths which could be used to intervene in the process of post-exposure carcinogenesis, and that cancer risks at low doses could be described as days or weeks of life lost.