Desmoplastic trichoepithelioma, infiltrative/morpheaform BCC, and microcystic adnexal carcinoma: differentiation by immunohistochemistry and determining the need for Mohs micrographic surgery.

Several important cutaneous neoplasms present with basaloid cells in the dermis. Desmoplastic trichoepithelioma (DTE), infiltrative/morpheaform basal cell carcinoma (BCC), and microcystic adnexal carcinoma (MAC) are tumors in this category that may be difficult to differentiate, especially when evaluating thin biopsy specimens. An accurate diagnosis has important clinical implications. While DTE is a benign neoplasm with indolent behavior, infiltrative/morpheaform BCC and MAC can be highly aggressive, leading to substantial local destruction and potential metastasis. We present a patient with an unusual tumor demonstrating basaloid cells in the dermis and discuss the diagnostic approach for these lesions, emphasizing the potential role of cytokeratin 20 (CK20) in determining the need for Mohs micrographic surgery.