Background
The long-term efficacy of hypofractionated stereotactic radiotherapy (HSRT) for complex arteriovenous malformations (AVMs) remains uncertain. This study aimed to evaluate clinical and radiological outcomes following HSRT for AVMs.

Methods
A retrospective review was conducted of 59 patients treated with stereotactic radiotherapy between 2003 and 2020 at a tertiary center. Thirty-seven patients who received HSRT were included. AVM volumes were independently assessed by two radiologists using MRI. Radiological follow-up was available in 36 patients, clinical follow-up in 37. Treatment failure was defined as re-bleeding, re-treatment, or death.

Results
Median radiological follow-up was 4.6 years (range, 0.7–17.2), and clinical follow-up was 5.0 years (range, 0–18.0). Total obliteration was observed in 28 % (10/36) and partial obliteration in 39 % (14/36). Median AVM volumes were 3.8 cc (IQR 2.5–11.5) for total obliteration, 23.6 cc (IQR 11.1–45.1) for partial, and 17.1 cc (IQR 6.9–35.0) for no response (p = 0.004). Median biological effective dose (BED) was 146.4 Gy (IQR 73.0–146.4) for total obliteration, 54.1 Gy (IQR 35.2–56.4) for partial, and 54.1 Gy (IQR 28.4–56.4) for no response (p = 0.003). Treatment failure rates were 11 %, 36 %, 56 %, and 78 % at 2, 5, 10, and 15 years, respectively. Re-bleeding occurred in 0 % with total obliteration, 36 % with partial, and 30 % with no response (p = 0.13).

Conclusions
HSRT provided limited long-term efficacy in complex AVMs, with a high rate of treatment failure. Smaller AVM volumes and higher BED were associated with better outcomes.