Effect of Cobalt-60 Calibration Dose-Rate on Arteriovenous Malformation Obliteration after Stereotactic Radiosurgery

Abstract

Stereotactic radiosurgery (SRS) dose rate - how old the cobalt-60 sources are - is known to influence outcomes for some neurologic conditions and benign tumors, but it is not known if SRS dose rate influences arteriovenous malformation (AVM) obliteration. The objective of this study is to determine the association between cobalt-60 calibration dose rate and cerebral AVM obliteration in patients treated with SRS. Our hypothesis is that higher SRS dose rates are associated with increased incidence of cerebral AVM obliteration. We performed a retrospective study of 361 patients undergoing 411 AVM-directed SRS treatments between 2005 and 2019 at a single institution. Obliteration was confirmed with digital subtraction angiogram or MRI (if patient refused angiography). Lesion characteristics, SRS treatment details, and post-treatment obliteration and toxicities were recorded. Univariate and multivariate proportional hazards regression models of AVM outcomes with regard to SRS dose rate were performed, controlling for factors such as Spetzler-Martin Grade, maximum AVM extent, prior hemorrhage and prior embolization. At 10 years post-SRS, 68% of AVMs were obliterated on follow-up imaging. Dose rates >2.9 Gy/min were found to be significantly associated with AVM obliteration compared to those <2.1 Gy/min (p = 0.034, Gray's test). AVM size or Spetzler-Martin grade were also associated with obliteration, with obliteration more likely for smaller lesions, particularly those <3 cm in maximal diameter, or with lower Spetzler-Martin grade. Higher dose rates were not associated with development of post-SRS radiologic or symptomatic edema, though larger AVM volume was associated with both types of edema. Patients with cerebral AVMs treated with higher SRS dose rates (from fresh cobalt-60 sources) experience higher incidences of obliteration, without a significant change in the risk of post-treatment edema. These findings suggest that clinics offering SRS for AVMs should regularly renew cobalt-60 radiation sources to maintain high therapeutic dose rates.