Accurate diagnosis of metastatic tumors in the breast is crucial because the therapeutic approach is essentially different from primary tumors. A key morphological feature of metastatic tumors is their lack of an in situ carcinoma component. Here, we present a unique case of metastatic ovarian carcinoma spreading into mammary ducts and mimicked an in situ component of primary carcinoma. To our knowledge, this is the second case (and the first adult case) confirming the in situ-mimicking growth pattern of a metastatic tumor using immunohistochemistry.
A 69-year-old Japanese woman was found to have a breast mass with microcalcifications. She had a known history of ovarian mixed serous and endocervical-type mucinous (seromucinous) carcinoma. Needle biopsy specimen of the breast tumor revealed adenocarcinoma displaying an in situ-looking tubular architecture in addition to invasive micropapillary and papillary architectures with psammoma bodies. From these morphological features, metastatic serous carcinoma and invasive micropapillary carcinoma of breast origin were both suspected. In immunohistochemistry, the cancer cells were immunoreactive for WT1, PAX8, and CA125, and negative for GATA3, mammaglobin, and gross cystic disease fluid protein-15. Therefore, the breast tumor was diagnosed to be metastatic ovarian serous carcinoma. The in situ-looking architecture showed the same immunophenotype, but was surrounded by myoepithelium confirmed by immunohistochemistry (e.g. p63, cytokeratin 14, CD10). Thus, the histogenesis of the in situ-like tubular foci was could be explained by the spread of metastatic ovarian cancer cells into existing mammary ducts.
Metastatic tumors may spread into mammary duct units and mimic an in situ carcinoma component of primary breast cancer. This in situ-mimicking growth pattern can be a potential pitfall in establishing a correct diagnosis of metastasis to the breast. A panel of breast-related and extramammary organ/tumor-specific immunohistochemical markers may be helpful in distinguishing metastatic tumors from primary tumors.

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