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 carcinogenicc 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 1988 UNSCEAR Committee and the 1990 BEIR V Committee as the more appropriate basis 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 any neoplasm. 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 Sv of low-LET radiation, about 790 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 other than leukemia, since the linear-quadratic model applied to leukemia implies a DREF of about 2. If that population were exposed continuously to 1 mSv per year for an entire lifetime, about 560 extra cancer deaths would be expected to occur.