Tricholemmal carcinoma in continuity with trichoblastoma within nevus sebaceus.

A nodule arising within the nevus sebaceus on the vertex of the scalp of a 68-year-old woman was histopathologically and immunohistochemically investigated. We also used immunohistochemistry to investigate the cytokeratin (CK) distribution of the outer root sheaths of normal terminal hair follicles. The nodule consisted of two parts, a main exophytic part with a lobular proliferation and a small peripheral part with the features of trichoblastoma. The main exophytic lesion consisted of lobular aggregations composed of both or either basaloid cells and clear cells with the silhouette and cytology of malignancy. The columnar clear cells were aligned in a palisade at the periphery of the aggregations of clear cells, and the aggregations located in the superficial dermis were connected to the follicular infundibular structures. Almost all of the neoplastic aggregations were diffusely positive for CK7 (OV/TLR/30), and the innermost or inner cells of the neoplastic aggregations were positive for CK17; a similar staining pattern to that in the lower portion of the outer root sheath between the A and B fringes in normal terminal hair follicles. The exophytic part of the lesion was a malignant neoplasm with differentiation mainly toward the lower segment of the outer sheath between the A and B fringes of the terminal hair follicle, namely tricholemmal carcinoma. Our case may represent a collision of two distinctive neoplasms (tricholemmal carcinoma and trichoblastoma), however, an intimate relationship between these two neoplasms also should be considered.