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The exact cause of human breast cancer is unknown. Possibly several factors operate conjointlygenetic, environmental, viral, or hormonal. It is thus not surprising that cancer can arise in both breasts. In fact, when a woman has had cancer of one breast, her risk of developing one in the other is five times that of a woman who has never had mammary cancer.
Approximately 1% of patients with breast cancer are first seen with simultaneous bilateral cancers. These should be treated either by bilateral mastectomy at one operation or at least during the same period of hospitalization. Another group develop a cancer in the second breast months or years after their first mastectomy. Although conceivably these later lesions could be metastatic from the first, one should, in the absence of other known metastases, treat them optimistically as though they were new primary tumors. That the majority are new primaries is suggested
Hubbard TB. Prophylactic Mastectomy for Prevention of the Second Primary. JAMA. 1967;201(7):530. doi:10.1001/jama.1967.03130070050014