Evaluation of attributability of cancer to radiation exposure using probability of causation. Issues and perspectives

The probability of causation (PC) is known as a measure showing how much a disease after radiation exposure is attributable to the preceding exposure. PC is defined as a ratio of risk due to radiation exposure to a sum of baseline risk and the risk due to radiation exposure. For example, PC is calculated to be 50% where risk due to radiation exposure equals to the baseline risk, namely, in the situation where excess relative risk is 1. PC is a similar measure with those used in traditional epidemiology such as etiologic fraction, attributable fraction, attributable risk, etc. Recently, the term "assigned share" is more preferably used instead of PC mainly because PC is not exactly the probability, and because it is applied to the population not an individual. The basis for calculating PC is a cancer risk model developed by using epidemiological data of atomic bomb survivors in Hiroshima and Nagasaki. For this reason, the calculated PC values are dependent on extension of the follow-up period of A-bomb survivors, updates of cancer risk models including age and time modifying factors. Some of calculated PC values for selected cancer sites are shown here based on risk models derived from recent publications on atomic bomb survivors. (author)