Deterministic and stochastic effects associated with high-dose ionizing radiation (x-ray) exposure have been known for almost as long as ionizing radiation itself (1-3). At lower doses, radiation risks are primarily stochastic effects, in particular, somatic effects (cancer) rather than the deterministic effects characteristic of higher-dose exposure (4-6). In contrast to deterministic effects, for stochastic effects, scientific committees generally assume that at sufficiently low doses there is a positive linear component to the dose response—that is, that there is no threshold (4-6). This does not preclude there being higher-order (eg, quadratic) powers of dose in the dose response that may be of importance at higher doses. It is on this basis that models linear (or linear-quadratic) in dose are often used to extrapolate the experience of the Japanese atomic bomb survivors (who were typically exposed at a high dose rate to moderate doses [average, 0.1 Sv]) to estimate risks from low doses and low dose rates (4-6). Most population-based cancer risk estimates are based primarily on the Japanese atomic bomb survivor Life Span Study (LSS) cohort data (4-6). However, evidence of excess risks comes from a large number of other studies as well.