A contribution to the natural history of breast cancer. V. Bilateral primary breast cancer: incidence, risks and diagnosis of simultaneous primary cancer in the opposite breast.

Tissue from contralateral breast was taken from 505 patients with invasive breast cancer. Every 5th patient had simultaneous carcinoma in the opposite breast, 7.7% (39/505) had invasive cancer, and 13.1% (66/505) had carcinoma in situ. Invasive cancers in the opposite breast were diagnosed at an earlier stage as compared with the neoplasm of the primary breast. However, 26% of the patients already had positive axillary nodes. The following risk indicators of the bilateral disease were found: age at time of diagnosis, histologic type of tumours, familial breast cancer, suspicious mammography, P2/DY-breast parenchymal pattern (Wolfe) and multicentric cancer of primary breast. Our findings support the proposal to consider bilaterality of breast cancer as a sign of systemic disease of the breasts, involving the ductal and lobular system within eight quadrants of a paired organ (Ober).