AVM Radiosurgery: Where is the Volume Limit?

To evaluate the role of radiosurgery for large AVM (nidus volume > 8 ml). 200 cerebral AVM patients treated by Gamma Knife radiosurgery constituted the study database. Of them, 90 had a nidus volume > 8 ml. 38 of the 90 patients who fulfilled the following criteria were enrolled in result analysis. They were either treated more than 24 months ago or proved cured at any time earlier than 24 months. Grades of the AVM were II/2, III/11, IV/20, V/5 (Spetzler-Martin grading system). Integrated stereotaxic MR and stereotaxic x-ray angiography were used for treatment guidance. Volumes of the AVM nidi of the 38 patients were 8∼26 ml, median 11 ml. The prescribed mean maximum and minimum irradiation doses to the delineated AVM nidi were 36 Gy and 18 Gy, respectively. 21 patients were cured of AVM at 18∼36, mean 23 months, after radiosurgery. Almost complete obliteration occurred in 6 patients at 24∼46, mean 37 months, and subtotal obliteration occurred in 11 patients at 27∼60, mean 38 months, after radiosurgery. Nidus volume, not AVM grading, had an impact on the cure rate of AVM radiosurgery. T2-weighted high signals in the vicinity of the AVM were observed in 32 of 38 patients. Minor neurological deficits occurred in 3 patients. Re-bleeding occurred in 3 patients during the follow-up periods. Gamma Knife radiosurgery is currently an effective treatment alternative for large AVM, up to a nidus volume of 21 ml. The waiting time for complete obliteration of large AVM appears to be longer than for small ones. Nidus volume is the most decisive factor in radiosurgery of large AVM. Application of MR improves the conformity of treatment volume in AVM radiosurgery. The improvement minimizes the irradiation volumes and makes radiosurgery for large AVM safer.