Answer: Pigmented trichoblastoma

Histology: none provided

Discussion: Trichoblastoma typically presents as a slow-growing nodule on the head and neck, though additional anatomic sites, including the trunk and extremities may be affected. Trichoblastoma affects men and women with equal frequency, and, while it may occur at any age, the most common presentation is in the 5th to 7th decades. Histologically, trichoblastoma appears as a well-circumscribed nodular tumor that frequently extends through the entire dermis into the subcutaneous tissue. Individual tumor nests are composed of basaloid cells and demonstrate peripheral palisading, similar to basal cell carcinoma. Unlike basal call carcinoma, retraction artifact is not observed in trichoblastoma. The nests of trichoblastoma are surrounded by a densely cellular, condensed stroma. The stroma contains stellate and spindled fibroblasts that are sometimes associated with primitive hair follicle formation, known as “papillary mesenchymal bodies.” The pigmented variant of trichoblastoma contains melanin pigment within tumor cells, dendritic cells and macrophages. In this case, the tumor arose in association with an epidermal nevus. Lesions may also arise in association with nevus sebaceus.