ESTRO 2025 - Abstract Book

S1516

Clinical – Mixed sites & palliation

ESTRO 2025

Keywords: pain flare, SBRT

2332

Digital Poster Increased iso-toxic dose to the target in oligometastatic abdominal lymph nodes using CT-guided online adaptive SBRT Lucy A. van Werkhoven, Maaike T.W. Milder, Mischa S. Hoogeman, Remi A. Nout, Joost J. Nuyttens Radiotherapy, Erasmus MC Cancer Institute, University Medical Center, Rotterdam, Netherlands Purpose/Objective: We report dosimetric outcomes of a phase-II study in patients undergoing CT-guided, radiotherapy technologist (RTT) only, online adaptive stereotactic body radiotherapy (SBRT) on abdominal and pelvic lymph node oligometastases (A-P LN), to assess if this approach can iso-toxically increase the target dose. Material/Methods: Patients with oligometastatic A-P LN received 45 Gy in five fractions on the CyberKnife in the phase-II trial. For each patient, three plans were created using pre-treatment CT scans: Plan A, Standard of care (SOC) plan based on the planning CT prescribed to the 90% isodose-line; Plan B, adaptive plan with organ at risk (OAR) contours from a diagnostic CT and prescribed to the 90% isodose-line; Plan C , an adaptive plan, based on the planning CT prescribed to the 80% isodose-line. After a pre-fraction in-room CT scan on rails [1], the RTT used a decision tree to select the plan with the highest target coverage without exceeding OAR constraints (Figure 1). Dose volume histogram (DVH) parameters of the target and OAR were extracted from planning and fraction CTs to determine if the adaptive treatment increased the target dose without increasing dose to the OAR.