A 74-year-old woman presented with malignant progression of remnant epidermoid cyst manifesting as sudden onset of right ptosis and double vision. She had right oculomotor nerve paresis. She had a history of surgery for right cerebellopontine angle epidermoid cyst 20 years previously. T(1)-weighted magnetic resonance (MR) imaging demonstrated a hypointense mass lesion in the right cerebellopontine angle and basal cistern, and an isointense mass in the right paraclinoid region which was strongly enhanced. Diffusion-weighted MR imaging showed hyperintense areas in the right cerebellopontine angle, ambient cistern, and basal cistern, and the paraclinoid mass as hypointense. Surgery was performed using Dolenc's approach. Histological examination revealed that the paraclinoid tumor adjacent to the epidermoid tumor remnant was malignant transformation of epidermoid cyst into squamous cell carcinoma. She was treated with 46 Gy linac radiotherapy. She has been without tumor recurrence for 17 months. Malignant change of epidermoid cyst is extremely rare, but rapid progress of the symptoms suggests malignant transformation. MR imaging with gadolinium is useful for diagnosis.
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