Abstract

Sebaceous carcinomas are rare malignant neoplasms of sebaceous glands that arise within adnexal structures of the skin and commonly present on the eyelids, head, and neck. Proliferating pilar tumors (PPT) are uncommon neoplasms that are typically benign and arise from the outer root sheath of hair follicles. We present a case of a 49-year-old female with a 20+ year history of a stable posterior scalp/auricular lesion that unexpectedly started growing. The clinical differential diagnosis included benign vs. malignant proliferating pilar tumor, basal cell carcinoma, and squamous cell carcinoma. Histologic examination revealed predominately lobulated, bland-appearing squamous cells with trichilemmal keratinization. Notably, there were areas with cells that contained enlarged, basaloid, pleomorphic nuclei that were mitotically active, surrounded by faintly eosinophilic to focally vacuolated cytoplasm, and demonstrated a pagetoid growth pattern. Due to the dual nature of the neoplasm, this conjured a more concerning differential, prompting further workup. Immunohistochemical stains for SOX-10, Melan-A, and Cytokeratin 7 were negative. EMA and Adipophilin highlighted the atypical, vacuolated cells, favoring an associated sebaceous carcinoma. Interestingly, the constellation of histologic and immunohistochemical features was most consistent with a sebaceous carcinoma arising within a benign PPT. Thus, we confirmed a unique compound diagnosis that, at first glance, did not garner malignant concern