ESTRO 2025 - Abstract Book

S3096

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Department of Radiation Oncology (Maastro), GROW School for Oncology and Reproduction, Maastricht University Medical Centre+, Maastricht, Netherlands

Purpose/Objective: This study compares adaptive and non-adaptive proton therapy approaches in treatment planning for esophageal cancer patients, evaluating their impact on mean heart dose (MHD) and normal tissue complication probability (NTCP) for predicting mortality two years post-treatment. Material/Methods: A retrospective analysis was conducted on 41 esophageal cancer patients who received intensity modulated proton therapy (IMPT) and were adapted based on target coverage or MHD. Two treatment approaches were analyzed: adaptive and non-adaptive. In each approach, all fractions were assigned to the repeated weekly CT (reCT) closest in time to account for anatomical variations, using a robust, weighted dose accumulation. In the adaptive approach, all treatment plans, initial and adapted, were used for dose accumulation on an average CT, while in the non adaptive approach, only the initial plan was applied across all fractions. Dose metrics were analyzed for both approaches, including key target coverage metrics (ITV and GTV D 98% , ITV D max ) and doses to critical organs at risk (OARs), specifically MHD and mean lung dose (MLD). NTCP values for two-year mortality were calculated. Results: The analysis revealed notable differences between the adaptive and non-adaptive approaches. On average, the non-adaptive approach resulted in a 0.30 Gy higher ITV D 98% , 0.41 Gy higher ITV D max , and a 0.28 Gy lower GTV D 98% , likely due to the tendency to focus on reducing the MHD in the adaptive plans to account for inter-fractional anatomical changes. In both scenarios, all patients met GTV coverage goals, while 9 failed for the ITV. While the MLD difference only averaged a 0.02 Gy increase across all patients, the non-adaptive approach led to an average 0.62 Gy increase in MHD (1.04 Gy when considering only the 27 patients initially adapted due to a heart overdose). This increase in MHD was associated with a mean increase of 0.84 percentage points (pp) in NTCP for two-year mortality across all patients (p = 0.002), and a 1.44 pp increase when focusing on patients adapted for heart overdose concerns (p < 0.001).