Natural malignant transformation of an intracranial epidermoid cyst.

Malignant transformation of intracranial epidermoid cyst (EC) is very rare, and when it does occur, the clinical course is aggressive. We present an unusual case of natural malignant transformation of an intracranial EC, without a history of surgery. A 61-year-old woman was diagnosed with an EC in the right cerebellopontine angle and prepontine cistern 6 years before the operation described in this report. Her neurological symptoms deteriorated in the 2 months prior to the present admission. Magnetic resonance imaging revealed an irregular, nodular enhanced lesion in the cerebellopontine angle, prepontine cistern, and temporoparietal lobe. Subtotal resection of the lesion was performed. Pathological examination revealed malignant transformation of the EC. The patient died on postoperative day 36 from brainstem infarction, chemical meningitis, and hydrocephalus. Spontaneous rupture of the EC could have contributed to the malignant transformation. This could have been avoided by early removal of the EC.