ABSTRACT
A 52-year-old woman who had two episodes of cerebral hemorrhage due to cerebral arteriovenous malformation (AVM) had been treated with gamma knife radiosurgery twice. Complete obliteration of the AVM was confirmed after treatment and she was making steady progress. Twenty years after gamma knife radiosurgery, computed tomography showed cyst formation. The cyst grew larger with ensuing neurological signs and symptoms, so we performed surgery. Thereafter, it was possible to make a pathological assessment. Delayed cyst formation after gamma knife radiosurgery for AVM is known to be one of the late adverse effects of radiation but little is known beyond 10 years postoperatively. We report here on a case of cyst formation 20 years after gamma knife radiosurgery for AVM.