Trichilemmal cyst with carcinoma in situ within an atypical fibroxanthoma

A 91-year-old lady presented with an 8-month history of a slow growing keratotic nodule on the forehead, which was excised. Histological examination showed a well-defined lesion with marked cellular pleomorphism. Numerous mitoses were present. The cells stained positively with CD10 and CD68 and weakly with smooth muscle actin. However, the tumor was negative for cytokeratins (AE1/AE3), epithelial membrane antigen (EMA), S100, Melan A, HMB45, leukocyte common antigen (LCA), desmin, and CD31. Therefore, the lesion was designated as atypical fibroxanthoma (AFX) by exclusion. Within the AFX, there was also a cyst lined by squamous epithelium showing pilar keratinization. The epithelium showed full-thickness dysplasia with increased mitotic activity and was stained positively with AE1/AE3, thus supporting our view of carcinoma in situ within the wall of the cyst. To the best of our knowledge, this article describes the first case of trichilemmal cyst with carcinoma in situ arising within an AFX.