External auditory canal cholesteatoma

• Soft tissue density lesion in the external auditory canal on high-resolution bone window CT
• Focal osteonecrosis and bony erosion of the external auditory canal cortex
• Stage of disease: isolated EAC involvement versus middle ear, mastoid, or trans-temporal extension

• High-resolution bone window CT is the imaging method of choice, demonstrating soft tissue density material within the external auditory canal with sharp demarcation from surrounding tissues.
• Bony erosion and osteonecrosis of the external auditory canal cortex is a key distinguishing feature from keratosis obturans; focal bone necrosis with or without sequestration and lack of epithelial covering of the bony surface confirms the diagnosis.
• EAC cholesteatoma is classified into four stages: Stage I limited to EAC only; Stage II extends into tympanic membrane and middle ear; Stage III involves mastoid air cells in addition to EAC; Stage IV extends beyond temporal bone.
• The lesion appears as a well-circumscribed mass of soft tissue density, often with associated canal wall thinning, scalloping, or frank destruction of the bony canal cortex.
• MRI cannot reliably detect early compact bone involvement and is not routinely recommended for primary diagnosis or staging of external auditory canal cholesteatoma.

• Keratosis obturans: consists of epithelial debris without osteonecrosis or bone erosion; does not show focal osteonecrosis with sequestration and lacks lack of epithelial covering of bone.
• Necrotizing (malignant) otitis externa: presents with soft tissue inflammation and bone erosion but typically in immunocompromised patients; associated with more aggressive soft tissue involvement.
• Squamous cell carcinoma of the external auditory canal: may show similar bony erosion but typically presents as a more infiltrative soft tissue mass with irregular margins in older patients.
• Medial canal fibrosis: causes canal narrowing without the focal osteonecrosis and sequestration seen in cholesteatoma.

Report the precise location and extent of the soft tissue density lesion within the external auditory canal, clearly describe any focal osteonecrosis or bone sequestration, classify the stage (I–IV) based on extension into tympanic membrane/middle ear/mastoid/beyond temporal bone), and note the degree of canal wall erosion to guide surgical planning.

• Confusing external auditory canal cholesteatoma with keratosis obturans on imaging: the presence of osteonecrosis and bone erosion with exposed bone surface confirms cholesteatoma rather than keratosis obturans.
• Over-relying on MRI for diagnosis: MRI cannot reliably show early compact bone involvement and should not be used as the primary imaging modality; bone window CT is essential.
• Underestimating stage of disease on initial imaging: carefully assess for tympanic membrane involvement, middle ear extension, and mastoid air cell involvement to avoid understaging.
• Missing sequestration: carefully examine bone windows for small fragments of necrotic bone that may be difficult to see on soft tissue windows but are important for defining disease extent and treatment planning.