How to improve obliteration rates during volume-staged stereotactic radiosurgery for large arteriovenous malformations

OBJECTIVE The management of large-volume arteriovenous malformations (AVMs) with stereotactic radiosurgery (SRS) remains challenging. The authors retrospectively tested the hypothesis that AVM obliteration rates can be improved by increasing the percentage volume of an AVM that receives a minimal threshold dose of radiation. METHODS In 1992, the authors prospectively began to stage anatomical components in order to deliver higher single doses to AVMs >15 cm(3) in volume. Since that time 60 patients with large AVMs have undergone volume-staged SRS (VS-SRS). The median interval between the first stage and the second stage was 4.5 months (2.8-13.8 months). The median target volume was 11.6 cm3 (range 4.3-26 cm3) in the first-stage SRS and 10.6 cm(3) (range 2.8-33.7 cm(3)) in the second-stage SRS. The median margin dose was 16 Gy (range 13-18 Gy) for both SRS stages. RESULTS AVM obliteration after the initial two staged volumetric SRS treatments was confirmed by MRI alone in 4 patients and by angiography in 11 patients at a median follow-up of 82 months (range 0.4-206 months) after VS-SRS. The post-VS-SRS obliteration rates on angiography were 4% at 3 years, 13% at 4 years, 23% at 5 years, and 27% at 10 years. In multivariate analysis, only >= 20-Gy volume coverage was significantly associated with higher total obliteration rates confirmed by angiography. When the margin dose is >= 17 Gy and the 20-Gy SRS volume included >= 63% of the total target volume, the angiographically confirmed obliteration rates increased to 61% at 5 years and 70% at 10 years. CONCLUSIONS The outcomes of prospective VS-SRS for large AVMs can be improved by prescribing an AVM margin dose of >= 17 Gy and adding additional isocenters so that >= 63% of the internal AVM dose receives more than 20 Gy.