S-100 immunoreactivity in primary and metastatic carcinoma of the breast: a potential source of error in immunodiagnosis.

Immunoreactivity for S-100 protein, typically a marker for malignant melanoma and neural-derived tumors, was observed in neoplastic cells of 57 of 68 cases (84%) of formalin-fixed, paraffin-embedded primary and/or metastatic carcinoma of the breast of various histologic types. The extent of S-100 immunoreactivity varied, with only a minor proportion of positive tumor cells noted in some cases. An awareness of this staining profile for S-100 protein, particularly in metastatic poorly differentiated neoplasms with unknown primaries, is imperative for accurate immunohistochemical interpretation. Using a panel of reagents which includes antibodies to keratin proteins and epithelial membrane antigen, the epithelial nature of S-100-positive carcinomas may be readily defined. Tumor cells in all cases of primary and metastatic carcinoma of the breast evaluated in this study exhibited strong staining for both of these tissue markers. To preclude misinterpretation of tumor type due to anomalous staining patterns for a specific antibody, eg, S-100 protein, a panel of antibodies is recommended for assessment of metastatic poorly differentiated tumors.