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

Context.-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. Objective.-To characterize the expression patterns of CKs and p63 in BSLs. Design.-We evaluated the spindle cell component of 55 BSLs using CK 5/6, CK 903, CK MNF116, and p63. Results.-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. Conclusions.-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.