Nevus Sebaceous of Jadassohn With Secondary Cystic Papillary Hidradenoma.

Nevus sebaceous of Jadassohn (NS) is a congenital cutaneous hamartoma that typically presents at birth as a plaque on the face, scalp, or neck. While NS is associated with a risk of neoplastic transformation, the majority of secondary tumors are benign, with the most common being trichoblastomas and syringocystadenomas. In this report, we present a rare case of a 63-year-old male with a right parietal scalp NS with a secondary cystic papillary hidradenoma (HP), which is a benign tumor that almost exclusively occurs in the perineal region. Histopathologic examination was consistent with NS, revealing hyperkeratosis, parakeratosis, increased sebaceous glands, ectopic apocrine glands, and anomalous duct sweat gland hyperplasia. The lesion also demonstrated characteristics of HP, a neoplasm that has not been well-documented in association with NS. This case expands understanding of neoplasms that arise within NS and highlights histologic characteristics that differentiate HP from other neoplasms.