Rippled-pattern trichoblastoma with apocrine differentiation arising in a nevus sebaceus: report of a case and review of the literature.

A 36-year-old man developed a papular growth in a portion of a nevus sebaceus on the post-auricular scalp. Excision showed typical histologic changes of nevus sebaceus including epidermal papillomatosis with reduced numbers of hair follicles as well as numerous sebaceous glands high in the dermis that focally emptied directly to the overlying epidermis. Histologic sections of the papular growth at the superior pole of the nevus sebaceus showed a proliferation of cytologically bland basaloid epithelial tumor lobules both in the superficial dermis, with multiple connections to the epidermis, and within the deeper dermis in a nodular growth pattern demonstrating papillary mesenchymal bodies. Ductal structures with apocrine-type decapitation secretion were present. There was prominent palisading of nuclei in rows parallel to one another, alternating with bands of homogenous eosinophilic stromal material forming a ripple pattern resembling the Verocay bodies of schwannoma. The histologic features resembled those of rippled-pattern trichoblastoma with apocrine differentiation arising in a nevus sebaceus, an association not previously described. We discuss this case as well as review the literature on rippled-pattern trichoblastoma.