RISK-FACTORS FOR PRIMARY BREAST-CANCER IN JAPAN - 8-YEAR FOLLOW-UP OFATOMIC-BOMB SURVIVORS

Background. Findings from the Life Span Study (LSS) of the health effe cts of exposure to atomic bomb radiation have documented a strong dose -response relation between radiation exposure and breast cancer incide nce. Purpose. We analyzed data from the LSS cohort to identify nonradi ation risk factors for breast cancer and to determine whether these fa ctors were independent of the effects of radiation on breast cancer oc currence. Methods. Breast cancer incidence was ascertained among a coh ort of 22,200 residents of Hiroshima and Nagasaki, Japan, who had comp leted a mail survey between 1979 and 1981 to study nonradiation risk f actors for disease. During the subsequent follow-up period (average 8. 31 years), 161 cases of primary breast cancer were identified through population-based tumor registries in the two cities. Results. The risk of breast cancer was inversely related to age at menarche and weakly positive in relation to age at menopause and years of menstruation. A significant negative association of full-term pregnancy against breast cancer was observed, although the number of pregnancies beyond the fi rst was not related to the rate of breast cancer in the cohort. Women having their first full-term pregnancy before age 30 were at decreased risk of breast cancer relative to older women, but there was no trend . A nonsignificant, positive trend in risk was associated with increas ing weight and body mass (kg/m(2)). The risk of breast cancer among wo men with a history of estrogen use was 1.64 (95% confidence interval 1 .02-2.64) and with diabetes 2.06 (95% confidence interval 1.27-3.34). It was not possible to distinguish among additive and multiplicative m odels of the joint association of radiation dose and various non-radia tion-related exposures (age at menarche, full-term pregnancy, female h ormone preparations) identified in this analysis. Conclusions. Nonradi ation risk factors for breast cancer among Japanese atomic bomb surviv ors were consistent with those identified among other populations of w omen, although the prevalence of common risk factors was low Reproduct ive factors and hormone use appear to act independently of radiation e xposure on the risk of breast cancer among this population. (C) 1997 A cademic Press.