Giant Cystic Nodular Hidradenoma with Extensive Cystic Degeneration Presenting as a Knee Mass

Hidradenomas are benign adnexal neoplasms with eccrine or apocrine differentiation. They typically present as slow-growing, nodular, well-circumscribed lesions that may be solid, cystic, or mixed in composition. These tumors most commonly arise on the head, neck, or extremities, although they may occur at virtually any anatomical site. Hidradenomas are generally small, usually measuring up to 3 cm in diameter, and occurrence in the periarticular region is not uncommon, with reports of hidradenomas arising near joints such as the ankle, elbow, and knee documented in the literature. In this study, we report a case of a giant nodular hidradenoma with extensive cystic degeneration presenting as a progressively enlarging, painless knee mass after a traumatic injury. At 17.6 cm, this represents 1 of the largest hidradenomas reported in the literature, and highlights the diagnostic challenges posed by large cystic adnexal tumors arising in atypical periarticular locations.