The Carcinogenic Risks of Low-LET and High-LET Ionizing Radiations.

During the past decade, new and important information has become available concerning the carcinogenic effects of radiation and the implications for risk assessment and risk management. This new information comes mainly from further follow-up of the epidemiological studies of the Japanese atomic bomb survivors, patients irradiated medically for cancer and allied conditions, and workers exposed in various occupations. In the Japanese atomic bomb survivors the carcinogenic risks are estimated to be somewhat higher than previously, and this is due to the reassessment of the atomic-bomb dosimetry, further follow-up with.increase in the number of excess cancer deaths, particularly in survivors irradiated early in life, and changes in the methods of analysis to compute the age-specific risks of cancer. Overall, the cancer mortality data are now more compatible with the relative risk projection model. Because of the characteristics of the atomic bomb·survivor series as regards sample size, age and sex distribution, duration of follow-up, person-years at risk, and type of dosimetry, the mortality experience of the atomic bomb survivors was selected by the UNSCEAR Committee and the BEIR V Committee as the more appropriate ba,sis for projecting risk estimates for the general population. In the atomic bomb survivors, the dose-effect relationship for overall cancer mortality other than leukemia is consistent with linearity below 3 Gy, while the dose-effect relationship for leukemia, excluding chronic lymphatic leukemia, conforms best to a linear-quadratic function. The shape of the dose-incidence curve at low doses still remains uncertain, and the data do not rule out the possible existence of a threshold for ap.y neoplasm. The excess relative risk of mortality from all cancers combined is estimated to be 1.39 per Gy (shielded kenna), which corresponds to an absolute risk of 10.0 excess cancer deaths per 10,000 PYGy; the relative risk is 1.41 at 1 Gy (organ-absorbed dose), and an absolute risk of 13.07 excess cancer deaths per 10,000 PYGy. The BEIR V Committee developed modified multiplicative risk projection models to project lifetime risk estimates; the preferred models contained dose (and dose squared) terms as well as age at exposure, time since exposure, and interaction effects. In its report, it is estimated that if 100,000 persons received an instantaneous dose of 0.1 Gy of lowLET radiation, about 750 extra cancer deaths would be expected to occur during their remaining lifetime in addition to nearly 20,000 cancer deaths that will occur even in the absence of the radiation; a DREF of 2 or more should be applied to this estimate for cancers {)ther than leukemia, since the linear-quadratic model applied to leukemia implies a DREF of about 2. If that population were exposed continuously to 10 mGy per year for an entire lifetime, about 5,000 extra cancer deaths would be expected to occur. The BEIR V Committee concluded that the constanl additive risk model for risk estimation is no longer tenable; based on the modified multiplicative risk models for all cancers combined, the current risk estimate reported by the 1989 BEIR V Committee are appreciably higher, by a factor of about 1.5 to 2, than comparable estimates reported by the 1980 BEIR III Committee.