Nodular Hidradenoma Arising in a Nevus Sebaceus of Jadassohn

Nevus sebaceous of Jadassohn, a cutaneous hamartoma, results from disordered development of epidermal, follicular, sebaceous and apocrine differentiation that typically appears as a yellow verrucous plaque on the scalp or face. We present a 36‐year‐old white male with a congenital, nodular lesion along the hairline of his forehead, which had been traumatized repeatedly during hair grooming. Clinically, it was a well‐demarcated, curvilinear, erythematous, non‐tender plaque with no umbilication, ulceration or discharge. The periphery had a circumferential tan‐yellow, raised, waxy serpiginous border. Microscopically, the specimen had epidermal papillomatous hyperplasia with dilated infundibula of irregular mature sebaceous lobules, and mid‐dermal apocrine gland lobules. Within the hamartoma, was a well‐circumscribed, multi‐lobulated dermal tumor with epidermal and follicular attachments, focal ductal differentiation and cystic changes. Two distinct cell types were present; in the center were large polyhedral cells with focal clear cytoplasm, and at the periphery were spindle‐shaped cells with basophilic cytoplasm. These findings were typical of nodular hidradenoma arising within a nevus sebaceous of Jadassohn, a rare event with only seven cases previously documented in the literature.