Ionizing radiation is a well-established human carcinogen. The risk of developing radiation-induced cancer has traditionally been estimated from cancer incidence among survivors of the atomic bombs dropped in Hiroshima and Nagasaki in 1945. These data provide the best estimate of human cancer risk over the dose range from 20 cGy to 2.5 Gy for low-linear energy transfer radiation such as X or gamma rays. The cancer risk at doses below 10 cGy, however, is uncertain and has been the subject of controversy for decades. A major point of contention is whether the risk is linear as the dose decreases or displays a more complicated relationship. Both the International Council on Radiation Protection (ICRP) (1) and the United States National Council on Radiation Protection (NCRP) (2) have recommended using a linear no-threshold (LNT) model in extrapolating cancer risk from higher doses where more...

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