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 Sears to medica l radiation for tuberculosis (TB), The excess relative risk of breast cance r incidence in the Japanese atomic bomb survivors, however, is significantl y 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 1.0 5, 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 tw o data sets are statistically indistinguishable (two-sided P = 0.32). The b est estimate of the ratio between the excess absolute risk coefficients amo ng Japanese and Massachusetts women is 0.73 (95% CI 0.41, 1.44). After chil dhood 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 pat terns of risk over time since exposure between these groups exposed in chil dhood. However, in general there are no marked differences between the Mass achusetts and Japanese data sets in the age and time distribution of risk o f radiation-induced breast cancer. These data provide little evidence for a reduction of breast cancer risk after fractionated irradiation. (C) 1999 b y Radiation Research Society.