BACKGROUND: Radiosurgery is widely used to treat deep eloquent arteriovenous malformations (AVMs). OBJECTIVE: To evaluate how anatomic location, AVM size, and treatment parameters define outcome. METHODS: Retrospective analysis of 356 thalamic/basal ganglia and 160 brainstem AVMs treated with gamma knife radiosurgery. RESULTS: Median volume was 2 cm3 (range, 0.02-50) for supratentorial and 0.5 cm3 (range, 0.01-40) for brainstem AVMs; the marginal treatment doses were 17.5 to 25 Gy. After single treatment, obliteration was achieved in 65% of the brainstem, in 69% of the supratentorial, and 40% of the peritectal AVMs. Obliteration of lesions <4 cm3 was better in the brainstem (70%) and in the supratentorium (80%), but not in the peritectal region (40%). Complications were rare (6%–15%) and mild (≤modified Rankin scale [MRS] 2). Rebleed rate increased with size, but was not higher than before treatment. AVMs >4 cm3 in the brainstem were treated with unacceptable morbidity and low cure rate. Obliteration of large supratentorial AVMs was 65% to 47% with more complications ≥MRS3. Repeat radiosurgical treatment led to obliteration in 66% of the cases with minor morbidity. CONCLUSION: Deep eloquent AVMs <4 cm3 can be treated safely and effectively with radiosurgery. Obliteration of peritectal AVMs is significantly lower after a single treatment. However, morbidity is low, and repeat treatment leads to good obliteration. Radiosurgical treatment >4 cm3 in the brainstem is not recommended. Supratentorial deep AVMs >8 cm3 can be treated with radiosurgery with higher risk and lower obliteration rate. However, these lesions are difficult to treat with other treatment modalities, and a 50% success rate makes radiosurgery a good alternative even in this challenging group. ABBREVIATIONS: AVM, arteriovenous malformations MRS, modified Rankin scale
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