P13.08.A VOLUMETRIC REGRESSION OF MENINGIOMAS FOLLOWING SRS AND HFSRT: INSIGHTS FROM LONG-TERM ANALYSIS

Abstract BACKGROUND Meningiomas, the most common primary brain tumors in adults, are managed with surgical resection, stereotactic radiosurgery (SRS), or hypofractionated stereotactic radiotherapy (hFSRT). Despite high local control rates, the dynamics of volumetric tumor regression and radiobiological parameters remain underexplored. This study aims to evaluate these aspects in meningiomas treated with SRS and hFSRT, including the calculation of the α/β ratio to optimize radiotherapy strategies. METHODS A retrospective analysis of 151 patients with intracranial meningiomas treated between 2010 and 2021 was conducted. Volumetric analysis was performed on 62 lesions (1-10 cm³) treated with SRS (1×14 Gy) or hFSRT (3×7 Gy, 5×5 Gy, 5×6 Gy). Tumor volumes were assessed pre-treatment and at follow-up intervals using 3D MRI reconstruction. Radiobiological modeling and α/β calculation were performed using linear-quadratic (LQ) and linear-quadratic-linear (LQ-L) models. RESULTS SRS-treated meningiomas demonstrated significantly faster volumetric regression, shrinking by 32.7% of their original volume at 2 years and 67.36% at 4 years, compared to 15.32% and 31.66% for hFSRT (p = 0.003). The α/β ratio for meningiomas was calculated as 3.15 Gy (95% CI: 3.07-3.23 Gy), refining the understanding of their radiobiological characteristics. The overall local control rate was 91.33%, with comparable outcomes for SRS (91.43%) and hFSRT (91.30%). Complications included brain edema in 7.9% of patients and radiation necrosis in 2.6%. CONCLUSIONS CyberKnife SRS and hFSRT are effective and safe treatments for meningiomas, with SRS achieving significantly faster volumetric regression (19% vs. 9% annual reduction for hFSRT). The determination of the α/β ratio at 3.15 Gy provides new insights into meningioma radiobiology, facilitating personalized treatment strategies. Future research should investigate molecular and histopathological tumor subtypes in the context of volumetric changes.