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 cohortstudy was conducted on the occurrence of second primary cancersfollowing the first breast cancer in females. Of the 9, 503breast cancer patients newly diagnosed in the period 1965–1982who were followed up until the end of 1983 (average follow-upperiod, 5.7 years), 344 developed second cancers, whereas theexpected number had been 211 (relative risk (RR) = 1.6; 95%confidence interval (CI) = 1.5–1.8). The increased riskwas observed throughout the observation period, and was higherin patients of less than 45 years of age at diagnosis than inolder women. Significant excess risks were found for secondcancers 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 effectsof chemo- and radiotherapy administered for initial breast canceron the increased risk of the above mentioned second cancerswere also examined. These therapeutic measures were found notlikely to be related to the excess risks for cancers of thebuccal cavity, stomach and colon. For second cancer of the oppositebreast, however, both chemotherapy and radiotherapy remainedas possible risk factors. The effect of radiation was proposedas being a likely explanation for the excess risk of secondthyroid cancer.