Age at first primary as a determinant of the incidence of bilateral breast cancer. Cumulative and relative risks in a population‐based case‐control study

This epidemiologic investigation comprised 1351 of 1423 women in a defined geographic area consecutively diagnosed as having a primary breast cancer. Simultaneous bilateral disease occurred in only 1 patient, whereas a history of previous cancer in the contralateral breast was reported by 65 patients. This prevalence was related to that of 23 previous cases in an age‐matched control group of 1351 women from the same population. The relative risk of developing a second primary was 2.9 (95% confidence limit, 1.8–4.6) for the whole material and remained seemingly constant over several decades at a level predetermined by age at first diagnosis, namely 9.9 (95% confidence limit, 3.8–25.8) before the age of 50 and 1.9 (95% confidence limit, 1.1–3.2) after that age. The incidence ratio of bilateral to unilateral disease was used as an estimate of the lifetime risk of developing a second primary in this stable and well‐defined population. This calculation revealed cumulative risk figures of 13.3% and 3.5% for women younger and older than 50 years, respectively, at first diagnosis. It was concluded that the occurrence of bilateral disease, which reflects a multicentric neoplastic transformation of the breast epithelium, is a characteristic of early‐occurring (premenopausal) disease.