Cancer following medical irradiation.

Several generalizations about radiation carcinogenesis can be made: 1) a single exposure is sufficient to elevate cancer incidence many years later: 2) radiation-induced cancer cannot be distinguished from naturally occurring cancer, i.e., there is not unique radiogenic cancer; 3) all cancers appear to be increased after irradiation with the exception of chronic lymphocytic leukemia, and possibly Hodgkin's disease, cervical cancer, and a few others; 4) the breast, thyroid, and bone marrow appear especially radiosensitive; 5) leukemia is the most prominent radiogenic tumor and shows a wave-like pattern of excess incidence over time, and the excess begins within two to four years, peaks about six to eight years, and decreases to normal levels about 25 years later; 6) solid tumors have a minimum latent period of about ten years, and for several cancers, the temporal pattern of incidence appears to follow the natural incidence, i.e., the cancers do not occur before the ages normally associated with increased incidence, implying that age-dependent factors influence the expression of disease; 7) age at exposure is perhaps the most important host factor influencing subsequent cancer risk; 8) the percentage increase in cancer incidence per rad is not the same for all cancers, i.e., some cancer of high natural incidence, e.g., colon, have low "relative risks" and some cancers of low natural incidence, e.g., thyroid, have high "relative risks;" 9) dose-effect curves are often linear, but curvilinearity is also observed and is possibly associated with the need for "two ionizing events" for transformation to occur at low doses, the influence of cell sterilization at moderate doses, the likelihood of "wasted" dose at high doses, and/or the influence of factors that effect the expression of disease.