ESTRO 2025 - Abstract Book

S712

Clinical - CNS

ESTRO 2025

partnership between DKFZ and University Hospital Tübingen, Tübingen, Germany. 6 German Cancer Research Center, DKFZ, Heidelberg, Germany

Purpose/Objective: The 2023 ESTRO-EANO guideline 1 on glioblastoma radiotherapy (RT) target delineation has introduced a modified approach on the clinical target volume (CTV) margin, reducing the margin around the gross tumor volume (GTV) from 20 to 15 mm. This modification aimed to limit radiation exposure to surrounding healthy brain tissue while maintaining effective tumor control. In the present analysis we investigated the recurrence pattern in patients treated with reduced CTV margins. Hereby, we aimed to ensure that no increase in marginal recurrences is present in the up-to-date 15 mm CTV margins compared to the previous 20 mm CTV margins. Material/Methods: Since the implementation of the ESTRO-EANO guideline in 2023, our department has applied reduced CTV margins as the clinical standard RT protocol for glioblastoma patients. Eighteen months following this change, we conducted the first recurrence pattern analysis among these patients. We classified tumor recurrences by overlaying the initial tumor volume with the recurrence volume as identified on MRI and assessing the percentage overlap with the planning target volume (PTV). Recurrences were categorized as central, in-field, marginal, or distant, corresponding to >95%, >80–95%, 20–80%, and <20% overlap with the PTV, respectively, based on criteria defined by Chan et al. 2 Results: In this first interim analysis out of 60 primary RT glioblastoma cases, 24 were treated with normofractionated radiochemotherapy according to the EORTC 26981/22981-NCIC CE3 protocol 3 with reduced CTV margins of 15 mm. These 24 cases were examined for tumor recurrence in the follow-up MRI scans. Tumor recurrence was detected in 9 out of 24 patients, and recurrence pattern analysis was conducted for these cases. For these 9 patients, median age was 53 years at beginning of the RT. Histopathological examination revealed IDH wild-type in 9/9 patients, and MGMT promotor methylation was present in 2/9 patients. Tumor resection was performed in 6 patients, whereas 3 patients underwent stereotactic biopsy only prior to the start of radiation therapy. The recurrence pattern analysis revealed central recurrences in 6 of 9 cases, while 3 of 9 cases had in-field recurrences. Notably, no marginal or distant recurrences were observed in this analysis. Conclusion: The primary objective of this first real-world data analysis was to evaluate the recurrence patterns in the early phase following the CTV margin reduction as recommended in the ESTRO-EANO guideline. In this first interim recurrence pattern analysis, all cases showed either central or in-field recurrences and no marginal recurrences were detected.

Keywords: Glioblastoma, Recurrence, Margin

References: (1) Niyazi M, Andratschke N, Bendszus M, et al. ESTRO-EANO guideline on target delineation and radiotherapy details for glioblastoma. Radiother Oncol . 2023;184:109663. doi:10.1016/j.radonc.2023.109663 (2) Chan JL, Lee SW, Fraass BA, et al. Survival and failure patterns of high-grade gliomas after three-dimensional conformal radiotherapy. J Clin Oncol . 2002;20(6):1635-1642. doi:10.1200/JCO.2002.20.6.1635 (3) Stupp R, Mason WP, van den Bent MJ, et al. Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma. N Engl J Med . 2005;352(10):987-996. doi:10.1056/NEJMoa043330