ESTRO 2025 - Abstract Book

S669

Clinical - CNS

ESTRO 2025

1204

Poster Discussion Local recurrence and radionecrosis after single-isocenter stereotactic radiotherapy for multiple brain metastases Jeroen A. Crouzen 1 , Jaap D. Zindler 1,2 , Mirjam E. Mast 1 , Jean-Paul J.E. Kleijnen 3 , Lia C. Versluis 1 , Masomah Hashimzadah 1 , Mandy Kiderlen 1 , Noëlle C.M.G. van der Voort van Zyp 1 , Marike L.D. Broekman 4,5,6 , Anna L. Petoukhova 3 1 Radiotherapy, Haaglanden Medical Center, The Hague, Netherlands. 2 Radiotherapy, HollandPTC, Delft, Netherlands. 3 Medical physics, Haaglanden Medical Center, The Hague, Netherlands. 4 Neurosurgery, Haaglanden Medical Center, The Hague, Netherlands. 5 Neurosurgery, Leiden University Medical Center, Leiden, Netherlands. 6 Cell and chemical biology, Leiden University Medical Center, Leiden, Netherlands Purpose/Objective: Stereotactic radiotherapy (SRT) is a common treatment for brain metastases (BMs). The single isocenter for multiple targets (SIMT) technique allows for faster and more efficient treatment of a large number of BMs concurrently. However, this approach may adversely affect planning target volume (PTV) coverage compared to conventional one isocenter per target methods. This is due to possible increased positioning uncertainties, especially for BMs located further away from the single isocenter. This study investigates the association of the tumor to isocenter distance with local recurrence and radionecrosis. Material/Methods: Patients treated with SRT using a SIMT were analyzed retrospectively. Previous cranial radiotherapy and inability to undergo MR imaging were exclusion criteria Radiation oncologists delineated gross tumor volume (GTV) based on T1-weighted MR imaging scans. PTVs were generated by expanding the GTVs by 1 mm. Treatments were delivered using a VersaHD LINAC with a 6D robotic couch. A non-coplanar volumetric modulated arc technique was used and plans were delivered using 6MV flattening filter free. Associations of potential risk factors with local recurrence or symptomatic radionecrosis were investigated with Cox regression analyses. A P value of <0.05 (two-sided) was considered statistically significant. All statistical analyses were performed using the software R version 4.3.1. Results: In total, 75 patients with altogether 357 BMs were included. The median number of irradiated BMs per patient was 3 (range 2-18). The median survival time after SRT was 9 months. Local recurrence occurred in 9% of patients and local recurrence-free survival was 89% after 18 months (Figure 1). Isocenter to tumor distance was not significantly associated with local recurrence (HR 1.02, 95%CI 0.71-1.46, P 0.92 ) in the univariable Cox regression analysis (Table 1). Symptomatic radionecrosis occurred in 13% of patients and radionecrosis-free survival was 85% after 18 months. Isocenter to tumor distance was not significantly associated with symptomatic radionecrosis in either the uni- or multivariable Cox regression analyses (HR 0.65, 95%CI 0.41-1.02, P 0.059 ). Tumor volume was significantly associated with local recurrence and symptomatic radionecrosis after correction for other factors.