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 MNF1...