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.
Description
There is a well-demarcated but noncircumscribed basophilic nodule in the superficial dermis. The basaloid cells form nests, some with structure, and are embedded within a condensed and hypercellular stroma. Pigment is present within the structures and the stroma.
