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Impact of PET-imaging during treatment planning on outcome in meningioma patients.

e13542 Background: Modern radiotherapy (RT) techniques such as IMRT combined with IGRT increased safety and precision over the years. However, during treatment planning, the definition of the planning target volume (PTV) remains challenging, and differentiation between healthy tissue, i.e., meninges, post-operative changes, and residual tumor can be difficult using MR and CT imaging alone. In this study, we evaluated the impact of additional PET-imaging on local control (LCR) and overall survival (OS). Methods: We analyzed 351 patients with primary RT of meningiomas treated between 1996 and 2018 and divided the cohort into low-grade (n = 283) and high-grade (n = 68) cases. All patients were treated with fractionated stereotactic radiotherapy (FSRT) with a median dose of 54.0 Gy and a median single dose of 1.8 Gy. A radiation oncologist delineated PTV based on diagnostic CT and MRI and, if available, additional PET-imaging. We used only PET-images acquired within 50 days before RT. In our clinic, PET-planned meningioma treatment started in 2000 with Methionine (2001-2010), between 2004 and 2011 F-18 FET tracer was used, and since 2011 only 68Ga-Dotanoc/Dotatoc PETs are acquired. This study is registered under the open science framework: DOI 10.17605/OSF.IO/RYX9D. Results: Median follow-up was 6.9 years (95%-KI: 6.3-7.4). For low-grade meningiomas, mean OS was 15.5 years (95%-KI: 14.7-16.2) and mean PFS was 15.7 years (95%-KI: 14.9-16.6); for high-grade cases, median OS was 13.8 years (95%-KI: 10.4-17.1), and median PFS was 8.9 years (95%-KI: 6.4-11.4). PET imaging had a significant impact on OS (p = 0.030) and PFS (p = 0.023) for low-grade meningiomas; however, in the multivariate analysis (with the prognostic factors age, gender, PTV, Karnofsky index, and time from resection to RT), it remained only significant for LCR. For high-grade cases, PET-imaging had no influence. Conclusions: PET-imaging improves the detection of tumor cells, especially during treatment planning. It showed a significant influence on OS and LCR. Further analyses will investigate the influence of PET regarding, e.g., residual tumor tissue, tumor size, and establish cut-off values for which tumors additional PET-imaging might be beneficial. With the further prognostic a weighted scoring system will be developed for prognostic assessment. [Table: see text]