Cerebellopontine Angle Epidermoid Cyst With Malignant Transformation Into Squamous Cell Carcinoma: An Unusual Complication of a Benign Intracranial Tumor.

Epidermoid cysts (ECs) are typically located in the posterior cranial fossa, most commonly at the cerebellopontine angle (CPA). Although generally benign, the cyst's epithelial lining can undergo malignant transformation into squamous cell carcinoma (SCC), a rare occurrence associated with poor prognosis. We present the case of a 55-year-old woman with a history of migraines who was incidentally diagnosed with a right CPA lesion consistent with an EC at the age of 30. Twenty-five years later, she developed progressive right-sided hearing loss, tinnitus, vertigo, headache, dysgeusia, and gait disturbances. Histopathological examination confirmed malignant transformation of the EC into SCC, supported by immunohistochemical markers including p63, p40, CK5/6, and D2-40. She was treated with fractionated stereotactic radiotherapy, carboplatin-paclitaxel chemotherapy, and the immune checkpoint inhibitor pembrolizumab, demonstrating good treatment tolerance and clinical stability.