Comparison of breast cancer incidence in the Massachusetts tuberculosis fluoroscopy cohort and in the Japanese atomic bomb survivors.

Breast cancer has occurred in excess among women exposed briefly to atomic bomb radiation and among those exposed repeatedly over many years to medical radiation for tuberculosis (TB). The excess relative risk of breast cancer incidence in the Japanese atomic bomb survivors, however, is significantly higher (two-sided P = 0.04) than that in the Massachusetts TB fluoroscopy patients. The best estimate of the ratio between the excess relative risk coefficients for the Japanese and Massachusetts cohorts is 2.11 (95% CI 1.05, 4.95). However, this higher relative excess risk is attributable to the lower baseline risk of breast cancer among Japanese women compared with the Massachusetts women, and the excess absolute breast cancer risks in the two data sets are statistically indistinguishable (two-sided P = 0.32). The best estimate of the ratio between the excess absolute risk coefficients among Japanese and Massachusetts women is 0.73 (95% CI 0.41, 1.44). After childhood exposures, an early onset of radiation-induced breast cancer was seen among Japanese atomic bomb survivors but not among the Massachusetts women. There are some indications (two-sided P = 0.04) of differences in the patterns of risk over time since exposure between these groups exposed in childhood. However, in general there are no marked differences between the Massachusetts and Japanese data sets in the age and time distribution of risk of radiation-induced breast cancer. These data provide little evidence for a reduction of breast cancer risk after fractionated irradiation.