We report a rare case of adenoid cystic carcinoma (ACC) arising from the external auditory canal (EAC), which was preoperatively misdiagnosed as pleomorphic adenoma. A 69-year-old woman presented with a 10-year history of left-sided hearing loss and repeated episodes of otorrhea. Her left EAC showed marked swelling over the entire circumference, with severe mixed hearing loss. Computed tomography revealed a soft-tissue shadow filling left EAC without bone destruction and an intact middle ear cavity. Histopathological examination of a biopsy specimen revealed the diagnosis of pleomorphic adenoma, and transcanal tumor resection was performed. However, histopathological examination of the surgical specimen revealed the diagnosis of ACC, clinical stage T2N0M0. The patient then underwent lateral temporal bone resection, followed by intensity-modulated radiotherapy. Her post-treatment course has remained uneventful over the 27 months from the start of treatment, with no evidence of recurrence. It is important to be aware of the necessity of several repeat histopathological examinations if the biopsy sample reveals a rare diagnosis.
