Malignant transformation of an epidermoid cyst in the cerebellopontine angle.

Intracranial squamous cell carcinoma is extremely rare, with most of the cases arising from malignant transformation of an epidermoid or a dermoid cyst. The patient presented with facial weakness. Initial magnetic resonance imaging revealed a mass in the right cerebellopontine angle. A subtotal resection was performed via right retrosigmoid suboccipital approach. Histopathological findings were consistent with an epidermoid tumor. Five months later, the patient underwent gamma knife radiosurgery due to highly probable recurrent epidermoid tumor. Two years after, the patient's neurological deficit had been newly developed, and follow-up magnetic resonance imaging demonstrated a large contrast-enhancing tumor in the left cerebellopontine angle, which compressed the brainstem. After resection of the tumor, histopathological examinations revealed a squamous cell carcinoma probably arising from an underlying epidermoid cyst. We report a case of an epidermoid tumor in the cerebellopontine angle that transformed into a squamous cell carcinoma.