ESTRO 2025 - Abstract Book

S724

Clinical - CNS

ESTRO 2025

impact of radiation doses while adjusting for interactions with Gross Tumor Volume (GTV) and Whole Brain Radiotherapy (WBRT). Various criteria were used to differentiate tumor progression and radiation necrosis, including the evaluation of serial MRIs, perfusion MR, MR spectroscopy, and, in some cases, surgical pathology. Results: The study included 340 brain metastases from 165 patients. 129 lesions were treated with 20 Gy (37.9%), 183 with 22 Gy (53.8%), and 28 with 24 Gy ( 8.2%). The mean individual GTV was 0.50 cc. The median follow-up was 20 months. Lesions treated with 20 Gy had a mean GTV of 0.75 cc, compared to 0.33 cc for the 22 Gy group and 0.41 cc for the 24 Gy group (P < 0.0001). Local control rates adjusted for GTV volume at one year were higher in the 22 Gy group (95.1%) compared to the 20 Gy group (89.0%) (P = 0.0264), Nonetheless, 24 Gy was not associated with statistically significant improvement in local control compared to 22 Gy (P = 0.12). A significant association was found between radiation dose and radiation necrosis risk (P = 0.004), although logistic regression indicated that GTV was positively associated with increased radiation necrosis risk (P = 0.0085, OR = 1.59). Prior WBRT was associated with lower local control rates (P = 0.0242). Conclusion: This study demonstrates that SRS at 22 Gy provides superior local control for metastatic brain lesions compared to 20 Gy, with no statistically significant difference between 22 Gy and 24 Gy. Larger GTV volumes were associated with a higher risk of radiation necrosis. Additionally, prior WBRT was linked to lower local control rates. More trials are needed to find the optimal SRS doses to maximize local control while minimizing radiation necrosis risk. Poster Discussion Intraoperative Radiotherapy after surgical resection of Brain Metastases: Feasibility and Efficacy. NCT04847284 Palmira Foro 1 , Francesco Amorelli 1 , Maria Lopez 2 , Maria Martinez-Garcia 3 , Jaume Juan 4 , Gloria Villalba-Martinez 2 1 Radiation Oncology, Hospital del Mar, Barcelona, Spain. 2 Neurosurgery, Hospital del Mar, Barcelona, Spain. 3 Medical Oncology, Hospital del Mar, Barcelona, Spain. 4 Statistical, IDIAP Jordi Gol, Barcelona, Spain Purpose/Objective: Intraoperative radiation therapy (IORT) is an alternative to local radiation therapy for brain metastases (BM) but the most effective dose is unknown. The objective of this study is to evaluate the efficacy and safety of IORT in patients with surgical excision of BM at a dose of 20Gy. Material/Methods: This Phase II single-institution, open-label, non-randomized trial evaluates IORT immediately after the resection of BM. The first analysis conducted on 12 patients enrolled up to July 2024. The study aims to recruit a total of 25 patients. Inclusion criteria include a KPS score ≥70, brain injury on MRI maintaining at least 1cm distance from the optic-nerve/chiasm and brainstem, with no contact with the dura-mater. Surgery uses neuronavigation to confirm metastasis and rule out primary CNS tumors, lymphomas, SCLCs or germinomas. IORT, delivered via the INTRABEAM-system, uses applicators (1.5–5.0cm) tailored to the cavity geometry. The total dose is 20Gy at 2cm from the applicator-surface, with a maximum of 8Gy allowed for at risk structures (optic-nerve/chiasm and brainstem). Cases not meeting inclusion criteria or unable to receive the prescribed dose are classified as screening-failures. Follow-ups include MRI 72h post-IORT, cognitive-testing, and MRI at 6weeks, then every 3months. Keywords: SRS, Radiation Necrosis, Dose 3376