[Cyst formation after radiosurgery for brain arteriovenous malformation treated with cystoperitoneal shunt].

Gamma knife and CyberKnife radiosurgery are well established and less invasive treatments for arteriovenous malformation. Delayed cyst formation is a rare but well-known complication of radiosurgery for arteriovenous malformations. The optimal treatment of cysts forming after radiosurgery remains debatable. We present a case of cyst formation after radiosurgery for brain arteriovenous malformation that was treated with a cystoperitoneal shunt (C-P shunt). A 36-year-old woman presented with left hemiparesis and numbness. Computed tomography (CT) revealed intracranial hemorrhage in the right basal ganglia. Digital subtraction angiography revealed arteriovenous malformation in the brain. Intravascular embolization was performed three times and radiosurgery was performed twice, whereby complete obliteration of the nidus was achieved. Six and a half years later, routine follow-up magnetic resonance imaging revealed cyst formation, and the patient gradually developed left hemiparesis. First, we performed stereotactic cyst aspiration. This initially resulted in a reduction in the size of the cyst and disappearance of left hemiparesis, but within a short time, the cyst increased in size again and there was recurrence of hemiparesis. Therefore, an Ommaya reservoir was established; aspiration of the cyst through this reservoir brought about an initial reduction in cyst size and alleviation of symptoms; however, no further reduction in cyst size or improvement in symptoms could be achieved. Twenty months after the placement of the Ommaya reservoir, we performed a C-P shunt operation. After the operation, further reduction in the cyst size and complete symptomatic recovery were observed.