[Diagnostic radiation and the risk of cancer]

The risk of radiation-related cancer following exposure to diagnostic radiation is of much concern. Diagnostic exposure is a repeated one to low dose radiation, while acute exposure occurred among atomic bomb survivors where the epidemiological survey contributes to the current cancer risk estimates of low doses. In several cohort studies on medical exposure at low doses, there is no statistical power of detection due to population size and no dose information. Even in cohort studies on occupational exposure there is no clear conclusion, however, a pooled analysis of nuclear workers in several countries is expected to produce a better basis for risk estimate at low doses. The risk estimate based on the linear non-threshold (LNT) dose response derived from the atomic bomb survivor data remains unresolved scientifically, and thus it has much uncertainty. Recent radiation biology suggests that a bystander effect and adaptive response might modify the estimated cancer risk based on the LNT model at low doses. However, there is no clear evidence in human data. The most effective way to clarify low-dose risk is to focus on the mechanism of radiation carcinogenesis. The risk from almost all diagnostic X-rays may be so small that no excess cancer incidence can be statistically detected.