Compactness index: a radiosurgery outcome predictor for patients with unruptured brain arteriovenous malformations

Abstract

OBJECTIVE The goal of the study was to define and quantify brain arteriovenous malformation (bAVM) compactness and to assess its effect on outcomes after Gamma Knife radiosurgery (GKRS) for unruptured bAVMs. METHODS Unsupervised machine learning with fuzzy c-means clustering was used to differentiate the tissue constituents of bAVMs on T2-weighted MR images. The percentages of vessel, brain, and CSF were quantified. The proposed compactness index, defined as the ratio of vasculature tissue to brain tissue, categorized bAVM morphology into compact, intermediate, and diffuse types according to the tertiles of this index. The outcomes of interest were complete obliteration and radiation-induced changes (RICs). RESULTS A total of 209 unruptured bAVMs treated with GKRS were retrospectively included. The median imaging and clinical follow-up periods were 49.2 and 72.3 months, respectively. One hundred seventy-three bAVMs (82.8%) achieved complete obliteration after a median latency period of 43.3 months. The rates of RIC and permanent RIC were 76.1% and 3.8%, respectively. Post-GKRS hemorrhage occurred in 14 patients (6.7%), resulting in an annual bleeding risk of 1.0%. Compact bAVM, smaller bAVM volume, and exclusively superficial venous drainage were independent predictors of complete obliteration. Diffuse bAVM morphology, larger bAVM volume, and higher margin dose were independently associated with RICs. CONCLUSIONS The compactness index quantitatively describes the compactness of unruptured bAVMs. Moreover, compact bAVMs may have a higher obliteration rate and a smaller risk of RICs than diffuse bAVMs. This finding could help guide decision-making regarding GKRS treatment for patients with unruptured bAVMs.

Vascular compactness of unruptured brain arteriovenous malformation predicts risk of hemorrhage after stereotactic radiosurgery

“…We could not determine whether the independent factors for post-SRS hemorrhage reflect the natural history of bAVMs or the effects of radiosurgery. However, previous analysis showed that bAVM volume and morphology significantly correlated with complete obliteration after radiosurgery 13 . Since obliteration of bAVMs substantially alter the hemorrhage risk, the predictors found in the current study less likely reflected the natural bleeding tendency, but could serve as a guidance for patient selection for management of unruptured bAVMs.…”

“…The independent effect of the margin dose could be due to faster obliteration process with higher radiation dose level, which shortened the latency period of bAVMs receiving SRS 30 , 31 ; alternatively, it could reflect that the whole bAVMs were completely covered in a certain level of radiation dose. However, previous studies have demonstrated a positive correlation between the incidence of radiologic or permanent RICs and prescribed dose 13 , 24 , 32 . The benefit of dose escalation should be weighed against radiation-induced side effects.…”

“…The aim of the current study was not to differentiate the hemorrhage risks between obliterated and non-obliterated bAVMs as patients with only 6 months of follow-up were included. However, a previous study, analyzing 209 unruptured bAVMs with at least 3 years of follow-up, demonstrated that diffuse, larger bAVMs with deep venous drainage have lower probability to achieve complete obliteration after SRS 13 . This could partly explain the independent effect of bAVM morphology (compact versus diffuse) and volume on post-SRS hemorrhage in the present analysis.…”

“…The voxels within the ROI were classified as vessel (flow-void), brain tissue (grey), and CSF (white) by the FCM algorithm based on the distribution of T2-weighted MRI signal intensity. The compactness index was previously reported to quantify the morphology (compact/diffuse) of bAVMs 13 . The index was defined as the ratio of the vascular volume to the brain tissue volume, which was equal to the ratio of vascular proportion (%) to the brain tissue proportion (%) within the ROI.…”

“…Previous studies have developed and validated an automated segmentation algorithm to differentiate vessel, brain and CSF components of bAVMs 11 , 12 . Compactness index, defined as the ratio of the vasculature proportion to the brain tissue proportion, was utilized to quantify the bAVM compactness 13 . In this study, we applied the algorithm and formula to investigate the impact of bAVM compactness on the occurrence of post-SRS hemorrhage in patients with unruptured bAVMs.…”