Considering metastatic tumor in the differential diagnosis is important when faced with a breast lesion that has unusual clinical, radiologic, gross, or microscopic features. The nonmammary primary lesion may be a new, occult neoplasm not evident to the patient’s physicians. The pre-operative clinical workup of an apparently healthy patient with a breast mass is often perfunctory and unlikely to exclude an occult malignant extramammary primary tumor. Even if a history of a previously treated nonmammary malignant tumor is known preoperatively by the surgeon, the information may not be communicated to the pathologist.

A lesion in the breast is the initial manifestation of a nonmammary malignant neoplasm in a minority of patients who have metastatic tumor in the breast. The occult primary tumor is usually a carcinoma, and one of the most common sites is the lung (1,2,3). The TTFI immunostain is positive in pulmonary carcinoma but not in mammary carcinoma. A surprising number of the occult lung lesions have been oat cell carcinomas (2,4). Other sites of occult, clinically inapparent neoplasms that have presented with metastases in the breast include the kidneys, stomach, ovaries (5,6), and intestinal carcinoid tumors (7,8). Previously diagnosed tumors that have given rise to metastases in the breast, sometimes rather late in the clinical course of the patient, include malignant melanoma (4,9), sarcomas (4), carcinoma of the lung (9,10), transitional cell carcinoma of the urinary bladder (11), and clear cell carcinoma of the kidney (12). Several types of malignant lymphoma have sometimes been included under the heading of metastases in the breast. However, in the breast these neoplasms are best regarded as primary tumors or as part of a systemic disease affecting the lymphoid system. A review of records for a 92-year period in one hospital revealed that about one-third of the nonmammary malignant tumors found in the breast originated from an occult tumor (13). Overall, the most common primary tumors that gave rise to mammary metastases were, in decreasing order of frequency, carcinoma of the lung, cutaneous malignant melanoma, carcinoma of the stomach, and clear-cell carcinoma of the kidney.

Adenocarcinomas of the gastrointestinal tract, especially the colon and rectum, are rarely the source of metastatic carcinoma in the breast, despite their relatively high frequency in the population at large (14). On the other hand, carcinoid tumors of the small bowel are a surprisingly frequent source of metastases in one or both breasts (15,16). Without knowledge of an extramammary primary, metastatic carcinoid tumor in the breast can be mistaken for a primary mammary carcinoma with endocrine differentiation (16) or for invasive lobular carcinoma.

Mammary metastases from medulloblastoma (17

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