ESTRO 2025 - Abstract Book

S656

Clinical - CNS

ESTRO 2025

contralateral normal-appearing white matter. On follow-up imaging, lesions were classified as stable or decreasing in size, persistently increasing, or fluctuating; t-test was used to compare rCBV values between these groups.

Results: The study cohort included 137 patients treated with IMPT to a median dose of 54 Gy (range 50.4-75 Gy RBE) in 30 fractions (range 28-41). Sixteen patients developed a total of 25 adequate RICE lesions; the median age was 57 years (range: 33-79), 56% were male and 56% Hispanic. Lesions were measured on up to 5 consecutive MRIs (median 5, range 1-5), yielding rCBV values in 74 instances. Median RICE lesion diameters at each of the 5 follow-up scans were 11.0, 12.5, 18.0, 13.4 and 12.8mm. During follow-up, the majority of lesions were classified as fluctuating (n=19), while others continuously decreased (n=5) or increased (n=1); all lesions except one decreased in the last follow up scan compared to their maximal size. Median mean rCBVs at each of the 5 follow-up scans were 0.60, 0.75, 0.95, 0.85 and 0.70; 99% and 75% of the lesions had mean rCBV <2.6 and <1.0, respectively. Median maximum rCBVs were 0.78, 0.96, 0.84, 0.93 and 0.95; 99% and 75% of the lesions had maximum rCBV <2.3 and <1.1, respectively. Although baseline median mean and maximum rCBVs were not different in decreasing lesions compared to fluctuating/increasing lesions, the overall values were significantly lower for the decreasing lesions (0.65 vs. 0.83, p=0.012 and 0.51 vs. 0.95, p=0.001, respectively). Conclusion: RICE lesions demonstrate mean and maximum rCBV values corresponding to published thresholds for adverse radiation treatment related effects; most were fluctuating, however lesions decreasing in size have lower rCBV values. DSC-MRI may serve as an adjunctive tool to help distinguish tumor recurrence from post-radiation changes. Digital Poster New enhancement following central nonenhancement as a radiological predictor of persistent growth of vestibular schwannomas following radiotherapy Masahiro Yamazaki 1,2 , Shigeyuki Takamatsu 1 , Takayuki Sakurai 1 , Masashi Taka 3 , Satoshi Kobayashi 1 , Eiichi Mizuno 2 1 Department of Radiology, Kanazawa University, Graduate School of Medical Sciences, Kanazawa city, Japan. 2 Department of Radiotherapy, Toyama CyberKnife Center, Toyama city, Japan. 3 Department of Radiotherapy, Toyama Prefectural Central Hospital, Toyama city, Japan Purpose/Objective: Some patients with vestibular schwannomas (VSs) face risk for brain edema or brainstem compression. Stereotactic radiotherapy is an established treatment method for VSs. However, there are few reports identifying radiological predictors of VSs following radiotherapy. Our recent findings suggest that the notch appearance can serve as a promising predictor of transient expansion and good outcome [1]. This study aims to identify a radiological predictor associated with poor outcomes following radiotherapy for VSs. Material/Methods: A total of 49 patients who received stereotactic radiotherapy and were subsequently monitored at our institution were subjected to image analysis. Based on the patterns of size change, the treated tumors were categorized into six groups, including those with consistent growth and transient expansion. Central nonenhancement (CNE) and newly enhanced regions on gadolinium-enhanced T1-weighted MRI were evaluated, as tumor vascularity can indicate the presence of viable cells. CNE is reported to be a radiological change following radiatherapy, which reflect subacute inflammation or vascular occlusion. Keywords: Proton therapy, late effects, rCBV 426