Fibrofolliculoma/Trichodiscoma With Focal Follicular Germinative Cell Differentiation

Fibrofolliculoma (FF) and trichodiscoma (TD) are benign adnexal tumors that arise from hair follicles or their surrounding structures. They seldom show a histology of hair germ. In this report, we present an interesting case of FF/TD with focal follicular germinative cell differentiation. A 56-year-old female presented with a solitary 8 mm nodule that was located behind the left ear. Histologically, a nodular tumor protruded from the skin surface and presented some follicular infundibulum-like structures that showed cystic expansion continuous with the epidermis. Thin epithelial cords consisted of 2 to 3 layers of cuboidal to columnar cells, and they radially expanded from a center of the follicle-like structure. The epithelial cords showed an anastomosed to fenestrated pattern with many various-sized sebaceous lobules. In some areas, peripheral portions of the epithelial cords continued to the follicular germs that were composed of basaloid cells with peripheral palisading. On immunohistochemistry, basaloid cells of the nest of follicular germinative cells only showed a strong expression of PHLDA1 and BerEP4. Based on these findings, we diagnosed this tumor as FF/TD with focal follicular germinative cell differentiation. It provides valuable insights into the origin and disease concept of these tumors.