Cancer and leukemia risks after low level radiation--controversy, facts and future.

Quantification of delayed low dose radiation (LDR) effects is still controversial. The current concept of the shape of the dose-response curve, particularly at the very low levels, is derived primarily by extrapolation from high doses and is affected by economic, social and political implications of cancer yield. Evidence based on epidemiological studies of populations exposed to fallout, occupational, intrauterine or background LDR is limited, due to methodological drawbacks and the need for extremely large sample sizes. Nevertheless, recent data indicate that LDR-induced childhood leukemia and thyroid cancer may exceed the rates predicted on the basis of the linear quadratic curve. The high yield in utero and in early childhood could be associated with low cumulative load of background radiation, and a consequently more effective radiation increment. A long term follow up of children exposed to 90 mSv after scalp X-irradiation revealed a relative risk of 3.8 and an excess risk of about 1.08 per 1000 man-sievert per year for thyroid cancer. Application of these findings to the post-Chernobyl state of events suggests that an increment of up to 20% in thyroid cancer might occur in a population exposed to 5 mSv as an aftermath of a similar accident. Prediction of future risk estimates should therefore be made with alertness and an open mind.