Recent Developments in Estimates of Cancer Risk from Ionizing Radiation

The probability of cancer induction is the main concern in the exposure of individuals to low doses of ionizing radiation. Estimates of the risk of cancer induction formerly (1977-1980) about 1 × 10^-2Sv^-1 have recently been increased to 3-5 × 10^-2Sv^-1 for low doses. These increased estimates are mainly the result of changes in the assessment of the Japanese survivors of the A-bombs in 1945. They result from the accumulation of 11 years or more of data on solid tumors, changes in the statistical projection methods and revisions in the dosimetry of those exposed. The dosimetry revisions are the result of a very comprehensive U. S.-Japan study which defines the doses more precisely than before. The high dose, high dose rate exposures in Japan result in risks higher than those due to low doses and dose rate. Thus these risks must be reduced by a dose rate effectiveness factor, which is derived mainly from laboratory information.

These changes in cancer risk estimates will impact on NCRP and ICRP recommendations on protection limits for low-LET radiation. For high-LET radiations such as those encountered in space the risks are known relative to low-LET radiation effects therefore a quality factor must be applied which introduces additional uncertainty.