Osteomas of the temporal bone are rare benign tumors that, in a small minority of cases, may give rise to secondary cholesteatoma. We report a case in which two independent pathological mechanisms appeared to coexist, resulting in a configuration not previously described in the literature. A 51-year-old woman presented with one year of right-sided otorrhoea and progressive mixed hearing loss unresponsive to antibiotic therapy. High-resolution computed tomography (HRCT) demonstrated a giant sessile osteoma arising from the anterior mastoid cortex with complete external auditory canal (EAC) obstruction. Non-echo-planar diffusion-weighted MRI (non-EPI DWI-MRI) identified cholesteatoma restricted to the middle ear and mastoid antrum, clearly distinct from chronically inflamed mastoid air cells. Intraoperatively, a near-intact tympanic membrane with a small attic retraction pocket formed the anterior boundary; no canal-wall cholesteatoma was present. Canal wall-down (CWD) radical mastoidectomy with osteoma excision and meatoconchoplasty achieved complete disease eradication. Histopathology confirmed compact osteoma and cholesteatoma. The cavity was dry and stable at the three-month follow-up. Upon review of the published case reports of temporal bone osteoma with associated cholesteatoma, no previously reported case combined a giant osteoma of the anterior mastoid cortex with cholesteatoma confined entirely to the middle ear and mastoid behind a structurally preserved tympanic membrane. The intraoperative finding of an attic retraction pocket suggests that Eustachian tube dysfunction may have played an independent contributory role alongside the osteoma. Combined HRCT and non-EPI DWI-MRI was essential for preoperative characterization and surgical planning.
