This 54 year old woman presented with a solitary plaque on her scalp. Clinically, there was a nevus sebaceus (not seen on the biopsy) with a prominent papule, which was biopsied. Trichoblastomas and other basaloid proliferations are commonly observed in association with nevus sebaceus. Some authors consider trichoblastomas to have low-grade malignant potential like a basal cell carcinoma, but the majority, if not all, are benign with a more hamartomatous quality. This is a typical example of a trichoblastoma, mimicking a BCC at low power. Trichoblastomas tend to have organoid or primitive follicular differentiation with a hypercellular and condensed stroma, and lack of peritumor clefting or significant atypia. Pigmentation is common, as in this case.
At higher power, these basaloid collections form primitive structures, with finger- or floret-like projections into the hypercellular stroma. There are scattered suggestions of papillary mesenchymal bodies, where the stroma sweeps into the basaoild structures. There is no signficant peritumoral clefting, atypia or mitotic activity. The cells are monomorphous and bland.