Second primary cancers following female breast cancer in Osaka, Japan--a population-based cohort study.

Using the data accumulated in the Osaka Cancer Registry, a cohort study was conducted on the occurrence of second primary cancers following the first breast cancer in females. Of the 9,503 breast cancer patients newly diagnosed in the period 1965-1982 who were followed up until the end of 1983 (average follow-up period, 5.7 years), 344 developed second cancers, whereas the expected number had been 211 (relative risk (RR) = 1.6; 95% confidence interval (CI) = 1.5-1.8). The increased risk was observed throughout the observation period, and was higher in patients of less than 45 years of age at diagnosis than in older women. Significant excess risks were found for second cancers of the opposite breast (RR = 4.2; 95% CI = 3.4-5.2), buccal cavity (RR = 3.6; 95% CI = 1.6-7.2), stomach (RR = 1.4; 95% CI = 1.2-1.8), colon (RR = 1.8; 95% CI = 1.1-2.1) and thyroid gland (RR = 3.2; 95% CI = 1.5-6.1). The effects of chemo- and radiotherapy administered for initial breast cancer on the increased risk of the above mentioned second cancers were also examined. These therapeutic measures were found not likely to be related to the excess risks for cancers of the buccal cavity, stomach and colon. For second cancer of the opposite breast, however, both chemotherapy and radiotherapy remained as possible risk factors. The effect of radiation was proposed as being a likely explanation for the excess risk of second thyroid cancer.