Keratin Expression Patterns in Stromal Cells of Benign Sclerosing Lesions of the Breast: A Potential Diagnostic Pitfall.

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Metaplastic carcinomas arising in association with benign sclerosing lesions (BSLs) are rare malignancies in which a neoplastic spindle cell proliferation can be recognized extending beyond the boundaries of the complex sclerosing lesion or papilloma. However, in cases in which the metaplastic carcinoma is of the low-grade fibromatosis-like type or is a low-grade adenosquamous carcinoma, distinction from the background BSL can be a significant challenge. Cytokeratin (CK) and/or p63 immunostains are helpful in confirming the diagnosis of metaplastic carcinoma, but the expression patterns of these markers in the stromal cells of BSLs have not been well characterized. To characterize the expression patterns of CKs and p63 in BSLs. We evaluated the spindle cell component of 55 BSLs using CK 5/6, CK 903, CK MNF116, and p63. A total of 45 cases (81%) showed no staining for CKs or p63 in benign stromal cells. CK 5/6, CK 903, and p63 were positive in one case each. CK MNF116 stained spindle cells within 10 BSLs. No cases showed spindle cell reactivity for all 4 markers. Positive cases demonstrated very focal, weak staining of spindle cells; only 1 case showed focal, moderate CK staining. Spindle cell positivity was not associated with lesion type, growth pattern, spindle cell atypia, or mitoses. These findings suggest that although the presence or absence of expression of CK 5/6, CK 903, and p63 may be useful to distinguish BSL from metaplastic carcinomas arising in this setting, CK MNF116 positivity may be a diagnostic pitfall.