Malignant transformation eight years after removal of a benign epidermoid cyst: a case report.

Malignant transformation of benign epidermoid cysts is rare and their prognosis remains poor. A 56-year-old woman presented with left facial hypoesthesia and photophobia in the left eye. She had undergone removal of a benign epidermoid cyst in the cerebellopontine angle 8 years previously. Magnetic resonance imaging of the brain revealed a cystic lesion in the left cerebellopontine angle. The cyst wall was enhanced by gadolinium-DTPA. She underwent removal again and the histopathologic diagnosis was squamous cell carcinoma. Gamma knife radiosurgery was performed on the remnant lesion with a marginal dose of 15 Gy. The tumor shrank rapidly for 2 months after radiosurgery, but recurred 9 months later. She underwent radiosurgery again with a marginal dose of 12 Gy. A gradual increase in the size of the enhanced lesion was seen during the 4-month follow-up period subsequent to the second radiosurgery. Careful observation, employing serial magnetic resonance images, is necessary for incompletely resected epidermoid cysts because intervals before malignant transformation reportedly range from 3 months to 33 years. Newly identified contrast enhancement strongly indicates malignant change in epidermoid cysts. Gamma knife radiosurgery may be useful for short-term control of intracranial squamous cell carcinomas, but long-term effects are presently unknown.