Hypofractionated Stereotactic Radiotherapy for the Treatment of Benign Intracranial Meningiomas: Long-Term Safety and Efficacy

Introduction: Hypofractionated stereotactic radiotherapy (hSRT) has emerged as an alternative to single-fraction stereotactic radiosurgery (SRS) and conventionally fractionated radiotherapy for the treatment of intracranial meningiomas (ICMs). However, there is a need for data showing long-term efficacy and complication rates, particularly for larger tumors in sensitive locations. Methods: A retrospective review was conducted on adult patients with ICMs seen at a tertiary care center. Eligible patients were treated with the CyberKnife platform and had a planned treatment course of 3–5 fractions from 2011–2020. The local control was assessed based on radiographic stability and the late toxicity/radionecrosis rates were recorded. Radiographic progression-free survival (PFS) and overall survival (OS) were estimated using the Kaplan–Meier method. Results: In total, 62 patients (age 26–87) with 67 treated tumors were included in this study with a median follow-up of 64.7 months. RT was delivered as the primary treatment in 62.7% of cases and for recurrence in 37.3%. The most common tumor locations were the convexity of the brain and the base of the skull. The tumor sizes ranged from 0.1–51.8 cc and the median planning target volume was 4.9 cc. The most common treatment schedule was 18 Gy in 3 fractions. The five-year PFS and OS were 85.2% and 91.0%, respectively. The late grade III/IV toxicity rate was 3.2% and the radionecrosis rate was 4.8%. Conclusions: Based on our data, hSRT remains an effective modality to treat low-grade ICMs with acceptable long-term toxicity and radionecrosis rates. hSRT should be offered to patients who are not ideal candidates for SRS while preserving the benefits of hypofractionation.

P04.15 Radiomics features in prediction of pseudo-progression and response to hypofractionated stereotactic radiotherapy and anti-PDL1 Durvalumab combination in recurrent glioblastoma from STERIMGLI phase I trial

BACKGROUND Due to the risk of pseudo-progression, evaluating the response of such combined treatment with anti-PDL1 immunotherapy and hypofractionated stereotactic radiotherapy (hFSRT) in patients with recurrent glioblastoma (GBM) is difficult even with iRANO criteria. We aim to analyze multi-modal MRI radiomic features in recurrent GBM patients included in the phase I STERIMGLI (NCT02866747) clinical trial. MATERIAL AND METHODS In phase I trial STERIMGLI, six patients received hFSRT 24 Gy in 3 fractions of 8 Gy prescribed to the 80% covering isodose in combination with Durvalumab IV 1500mg the same day following the third fraction of radiation then every 4 weeks for a maximum of 12 months. All patients underwent multi-modal MRI acquisitions. Contrast-enhancement on post-gadolinium T1 MRI (Gd-T1) and hyper-T2 signal on FLAIR MRI were manually segmented as volume-of-interest (VOIs) from baseline at week 0 (W0) and every eight weeks until weeks sixteen (W16). For patient 05, segmentation was performed until progression (W40). First order radiomics were extracted and normalized from these longitudinal VOIs in a total of 42 MRI: volume (in cm3), mean and median intensity, entropy, skewness and kurtosis were computed for each VOI. In total 504 (2 x 42 x 6) features were extracted. RESULTS Two patients underwent pseudoprogression (patients 04 and 05). All patients but one (patient 05) presented an early relapse with an increase of FLAIR and Gd-T1 volumes and respective entropies from the first evaluation without decrease during follow up (until W16). Patient 05 presented a Dose Limiting Toxicities and had the longest time to progression after Durvalumab and hFSRT. Contrary to the other patients, his FLAIR radiomics features presented a decrease of volume, intensity and entropy between W0 and W16 then, interestingly an increase between W24 and W40, date of progression. Moreover, patient 05 presented the lowest tumor volume and the lowest Gd-T1entropy. The intensity of the Gd-T1 followed the same trend as the hyper intense signal on the FLAIR in all patients, in particular for patient 05. CONCLUSION In our study, entropy and tumor volume for both FLAIR and Gd-T1, and FLAIR intensity seems to be the most interesting parameters to access the response of combined treatment. FLAIR signal may be more specific to the tumor microenvironment. First order radiomics allowed us to follow tumor heterogeneity and identify the patient with the longest time to progression.