A case of very large cyst formation with Gamma Knife radiosurgery for an arteriovenous malformation.

A 17-year-old male patient underwent Gamma Knife radiosurgery (GKRS) for a left parietal arteriovenous malformation (AVM), which presented with hemorrhage. The 15.0 cm3 nidus was covered with the 50% isodose. The maximum dose was 50 Gy and the margin dose was 25 Gy. Eleven months later he developed a right hemiparesis and MRI showed a large cyst. Cerebral angiography showed partial obliteration of the AVM nidus. Stereotactic removal of cyst fluid (about 70 cm3) was performed, and an Ommaya reservoir was inserted. Cyst formation after GKRS for cerebral arteriovenous malformation is a is side effect of radiosurgery about which we need to learn more.