Cyst Formation Following Gamma Knife Radiosurgery for Cerebral Arteriovenous Malformation

We retrospectively studied 15 patients, 9 men and 6 women aged 17 to 52 years (mean 28.1 years), who developed cyst formation following gamma knife radiosurgery (GKS) at our hospital for cerebral arteriovenous malformation (AVM). The mean nidus volume was 11 cm3 (0.1-26.7 cm3), and the mean prescription dose at the nidus margin was 20.0 Gy (18-28 Gy). Nidus obliteration was obtained in 9 patients, partial obliteration in 5, and no change in 1. Cyst formation was detected from 2.5 to 13.5 years (mean 6.4 years) after GKS. Three patients underwent craniotomy, and 2 received placement of an Ommaya reservoir. Spontaneous regression of cyst was observed in 2 patients. The outcome of the cyst was unknown in 2 patients, because of no response from the neurosurgeon the patients were referred to. Serial magnetic resonance imaging was performed in the other 6 patients because the cyst size was stable or asymptomatic. These findings suggest that cyst formation following GKS is not a "late complication." Placement of an Ommaya reservoir or cyst-peritoneal shunt is recommended for cysts with obliterated nidus. Craniotomy should be considered if the nidus is not completely obliterated or the cyst is associated with an expanding hematoma. Serial follow-up imaging is recommended for asymptomatic patients.