Atypical Nodular Hidradenoma of the Postauricular Region: A Report of a Rare Case With Radiological, Histopathological, and Immunohistochemical Correlation.

Atypical nodular hidradenoma is an uncommon adnexal neoplasm arising from eccrine sweat glands, with histological features intermediate between benign hidradenoma and hidradenocarcinoma. These tumors are rarely encountered in the head and neck region, particularly in the postauricular area, posing significant diagnostic challenges because of their nonspecific clinical and radiological appearance. We report the case of a 55-year-old male presenting with a slowly progressive postauricular swelling of four years' duration. Imaging demonstrated a well-defined, enhancing subcutaneous lesion without underlying bone or muscle invasion. Fine-needle aspiration cytology suggested a benign adnexal neoplasm. The patient underwent complete surgical excision with local flap reconstruction. Histopathological examination revealed an atypical adnexal tumor characterized by solid and cystic architecture, a dual cell population, focal infiltrative margins, and an absence of overt malignant features. Immunohistochemistry demonstrated a low proliferative index (Ki-67 approximately 1%), focal smooth muscle actin positivity, patchy S100 positivity, and negative p63 staining, confirming the diagnosis of atypical nodular hidradenoma. The postoperative period was uneventful, with no evidence of recurrence during follow-up. This report highlights the importance of complete surgical excision, histopathological examination, and immunohistochemistry in establishing the diagnosis and guiding management of this rare tumor.