OBJECTIVETo investigate the clinicopathologic characteristics and differential diagnosis of the metastases to the breast from non-mammary malignancies.

METHODSTwenty-eight cases were collected from 2004 to 2012;microscopic pathologic examinations and immunohistochemistry (EnVision method) were performed.

RESULTS(1) All except one patients were female, ranging from 16 to 77 years old (average 45.8 years). Twenty-six (92.9%) patients initially presented with the primary site lesions; while the other two (7.1%) patients initially presented with breast lesions. The mean interval from primary diagnosis to detection of metastatic breast lesions was 32 months (0-228 months). Fifteen patients (53.6%) had other metastases detected simultaneously or preceded the breast lesions. (2) Macroscopically, all the tumors were relatively circumscribed, with a mean diameter of 4.0 cm (0.6-12.0 cm). The histological types of the corresponding primary tumors were as follows: eight (28.6%) cases from lung adenocarcinoma, five (17.8%) from high-grade ovarian serous carcinoma, three (10.7%) from gastric adenocarcinoma, two (7.1%) from rectal adenocarcinoma, one (3.6%) from pancreatic neuroendocrine carcinoma, one (3.6%) from prostatic carcinoma, four (14.3%) from melanoma, and four (14.3%) from mesenchymal malignant tumors (three rhabdomyosarcomas and one epithelioid malignant peripheral nerve sheath tumor, MPNST). (3) Histologically, the metastatic tumors showed the morphologic characteristics of the primary tumors. Lymph-vascular invasion was observed in 19 cases. Immunohistochemical features of metastatic tumors were consistent with the primary tumors. Molecular markers for breast such as GCDFP15 and mammaglobin were negative. Metastatic tumors from lung adenocarcinoma expressed TTF-1 (8/8). Ovarian serous carcinoma metastases were positive for PAX8 (5/5) and WT1 (4/5). Gastric adenocarcinoma metastases were positive for CDX2 (3/3) and villin (1/3). Rectal adenocarcinoma metastases were positive for CDX2 (2/2). Pancreatic neuroendocrine tumor metastasis was positive for Syn and CgA (both 1/1). Prostate carcinoma metastasis was positive for AR, PSA and P504S (all 1/1). Melanoma metastases were positive for HMB45 (2/3) and S-100 protein (3/3). Rhabdomyosarcoma metastases were positive for vimentin, desmin and myoD1 (all 3/3). MPNST metastasis was positive for S-100 protein (1/1). (4) Follow-up data was available in 17 patients, with median follow-up time 54 months. The median survival from diagnosis to breast metastasis was 24 months.Seven of 17 patients died.

CONCLUSIONSMetastases to the breast from non-mammary malignancies are rare and show pathologic features of primary tumors. It is usually presumed to be a primary breast carcinoma. Histopathologic features and clinical history in conjunction with the immunohistochemical results should be considered in differentiating a secondary mass from a primary breast carcinoma.