The objective of the study was to assess the 10-year cumulative risk and clinical risk factors for the development of a contralateral cancer and to compare the tumours histopathologically. Among 1980 consecutive radically treated breast carcinoma patients a separate malignant breast tumour was diagnosed in 90 and 74 could be histopathologically compared with the primary tumour. The 10-year cumulative risk was 6.5% (95% CI: 5-8%). There was no difference in 10-year cumulative risk in developing a second breast tumour comparing premenopausal (7.1%) with postmenopausal women (6.1%). The cumulative risk among premenopausal tamoxifen-treated women (19.3%) or among patients with relapse (13.8%) was significantly increased as compared to similar patients without tamoxifen or without relapse. Sixty-six percent of the tumours displayed different histopathology. Morphologically similar and different tumours developed almost equally among patients with synchronous tumours and in those with or without relapse. We conclude that a radically treated breast cancer patient has a 10-year cumulative risk of 6.5% to develop a new malignant breast tumour. In premenopausal women the tumour-protective effect of two years tamoxifen application seems questionable. Histopathological comparison of the bilateral breast tumours enables discrimination of bilateral breast tumours as two primaries in 2/3 of the patients with morphologically different tumours.

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