NIMG-46. UNDERSTANDING PERMEABILITY CHANGES IN VESTIBULAR SCHWANNOMAS AS PART OF THE DYNAMIC RESPONSE TO RADIOSURGERY USING GOLDEN-ANGLE RADIAL SPARSE PARALLEL IMAGING, A RETROSPECTIVE STUDY

Abstract Vestibular schwannomas demonstrate different responses after stereotactic radiosurgery (SRS), commonly including a transient loss of internal enhancement on post contrast T1-weighted MRI thought to be due to an early reduction in tumor vascularity. We used dynamic contrast-enhanced based golden-angle radial sparse parallel (GRASP) MRI to characterize the vascular permeability changes underlying this phenomenon, with correlations to long-term tumor regression. Consecutive patients with vestibular schwannoma who underwent SRS between 2017-2019, had a transient loss of enhancement after SRS, and had long-term longitudinal GRASP studies (6, 12, and 24-36 months) were included in this retrospective cohort analysis (n = 19). Using GRAVIS, an analysis pipeline for GRASP studies, we extracted the key parameters normalized to the venous sinus from a region of interest within the tumor. The peak, area under the curve (AUC), and wash-in phase slope were significantly reduced at 6, 12, and 24 months after SRS (corrected p < 0.05), even while the internal enhancement returned in the tumors. Larger pre-SRS tumors were more likely to have a greater reduction in peak (p = 0.013) and AUC (p = 0.029) at 6 months. In a subset of patients (N = 13) with long-term follow-up, the median percentage reduction in tumor volume was 58% at median 62 months. These patients showed a strong correlation between changes in peak, AUC, and wash-in phase slope at 6 months and tumor volume at last follow-up. After SRS and loss of internal contrast uptake within the vestibular schwannomas, a slow vascular permeability dynamic found on GRASP studies persisted, suggesting the internal development of scar tissue. Moreover, the degree of reduction in GRASP parameters at 6 months positively predicted the degree of tumor regression long-term.