[Hemorrhage from an Angiographically Obliterated Arteriovenous Malformation after Gamma Knife Radiosurgery:An Immunohistochemistry Study].

A 24-year-old woman was diagnosed with a cerebral arteriovenous malformation(AVM)in the right parietal lobe(Spetzler-Martin grade I). The AVM was treated with stereotactic radiosurgery and was observed to have completely disappeared 3 years after radiosurgery. At the age of 35 years, the patient complained of a headache, and was referred to our hospital. A plain CT scan demonstrated a large cyst with niveau formation in the right parietal lobe. Cerebral angiography identified no recurrence of AVM. However, contrast MRI revealed an enhanced lesion on the surface of the cyst. The patient underwent cyst fenestration and total removal of the obliterated nidus through a right parietal craniotomy. Residual blood flow was confirmed in the obliterated nidus during surgery. The postoperative course was uneventful, and the headache was completely resolved. The patient was discharged without any neurological deficits. On pathological examination, a large number of small vessels were observed within the obliterated nidus. Immunohistochemistry demonstrated that these vessels were positive for CD31, CD34, and VEGFR-2, suggesting that endothelial progenitor cells may be involved in occult recurrence, cyst formation, and late bleeding after stereotactic radiosurgery targeting cerebral AVMs.