Cancer induction and non-stochastic effects.

Abstract

The mechanisms of carcinogenesis are not known in detail, but there is strong evidence that cancer usually arises from a single transformed cell. Hence, although the process of carcinogenesis appears to require a multiplicity of changes in the affected cancer-forming cell, such as may be associated with successive stages of tumour initiation, tumour promotion, and tumour progression, only one such change induced by radiation in an appropriate cell may be conceived to increase the probability of neoplasia in a suitably susceptible individual. For this reason, carcinogenic effects of radiation, like mutagenic effects of radiation, are considered for purposes of radiological protection to have no threshold and to behave as stochastic phenomena. In contrast, certain other effects of radiation, such as cataract of the lens, infertility, and depression of the bone marrow, require the killing of many cells in the affected organs. Thus, they vary in severity with the extent of cell loss and have thresholds of detectability which depend on the sensitivity with which the consequences of cell loss can be measured.