Temporal distributions of risk for radiation-induced cancers

Observations of cancer risk in irradiated human populations over time after exposure suggest that there are at least two, and perhaps more, very different patterns of temporal distribution of risk for radiation-induced cancer. The first, exemplified by bone sarcoma following therapeutic injection of 224Ra and chronic granulocytic leukemia in Japanese A-bomb survivors, is an early, wave-like pulse consisting of an increase in risk followed by a gradual decline back to baseline levels. The second, exemplified by breast cancer following a brief exposure to external gamma ray or X ray, and by lung cancer and stomach cancer in A-bomb survivors, is an increase in relative risk over about 10 years to a value which appears to remain constant over time thereafter. The first pattern suggests that tumor growth kinetics may play a central role in the temporal distribution of risk following exposure, while the second seems more consistent with multi-event models for carcinogenesis, in which radiation or some other cause of early events must be followed by one or more later events whose frequencies depend mainly on attained age. There are, however, other data that appear to conform to neither of the two models just mentioned. Influences of other cancer causes, like tobacco smoking, are potentially serious confounding factors in studies of induction period.