Modeling radioprotective mechanisms in the dose effect relation at low doses and low dose rates of ionizing radiation

A new model (Random Coincidence Model - Radiation Adapted (RCM-RA)) is prop osed which explains a possible pseudo threshold for stochastic radiation ef fects. It describes the formation of cancer in the case of multistep fixati on of lesions in the critical regions of tumor associated genes such as pro to-oncogenes or tumor-suppressor genes. The RCM-RA contains two different p ossibilities of DNA damage to complementary nucleotides. The damage may be caused either by radiation or by natural processes such as cellular radical s or thermal damage or by chemical cytotoxins. The model is based on the pr emise that radiation initially is bionegative, damaging organisms at their different levels of organization. The radiation, however, also induces vari ous cellular radioprotective mechanisms which decrease the damage by natura l processes. Considering both effects together, the theory explains apparen t thresholds in the dose-response relation for radiation carcinogenesis wit hout contradiction to the classical assumption that radiation is predominan tly bionegative at doses typically found in occupational exposures. (C) 199 9 Elsevier Science Inc. All rights reserved.