Retroauricular Hidradenocarcinoma Mimicking Basal Cell Carcinoma: Diagnostic Pitfalls and Histopathological Clues in Two Cases

Introduction: Hidradenocarcinoma is a rare malignant adnexal tumor characterized by aggressive behavior, high local recurrence, and metastatic potential. Due to its rarity and nonspecific presentation, it may mimic other cutaneous malignancies, particularly basal cell carcinoma (BCC) and squamous cell carcinoma, leading to delayed or inappropriate treatment. Case Presentations: We present two cases of retroauricular hidradenocarcinoma that clinically resembled BCC and had initially been interpreted as BCC on outside incisional biopsies. The first patient, a 63-year-old man, underwent wide excision with auricular cartilage resection. Histopathology demonstrated infiltrative solid-cystic epithelial neoplasia with marked atypia, clear cell change, comedo necrosis, and ductal differentiation. Immunohistochemistry showed diffuse CK5/6 positivity, focal BerEP4 and Bcl-2 positivity, and a Ki-67 index of 68–70%. Adjuvant radiotherapy was administered, with no recurrence during follow-up. The second patient, a 93-year-old man previously diagnosed with BCC elsewhere, presented with rapid tumor progression. Repeat biopsy revealed infiltrative clear cell neoplasia with marked pleomorphism, increased mitoses, focal necrosis, ductal differentiation. Computed tomography demonstrated temporal bone invasion. Immunohistochemistry supported hidradenocarcinoma with a Ki-67 index of 78–80%. Due to advanced disease and high surgical risk, helical intensity-modulated radiotherapy (helical IMRT) was initiated. Conclusion: Retroauricular hidradenocarcinoma may closely resemble BCC clinically and histopathologically. Recognition of infiltrative growth, ductal differentiation, high proliferative activity is critical for accurate diagnosis and multidisciplinary management.