ESTRO 2025 - Abstract Book

S3104

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Purpose/Objective: Oligometastases are increasingly treated with stereotactic body radiotherapy (SBRT), and online adaptive radiotherapy (OART) is often additionally implemented to prevent toxicity. This method takes significantly longer and is more demanding on the healthcare system, making effective resource implementation crucial. The aim of this retrospective in silico trial was to explore the use of OART for isotoxic personalized fractionation and in treatment dose adaptation for treating abdominal-pelvic lymph node (A-P LN) oligometastases. We hypothesized that this method could reduce the number of fractions, increase treatment capacity, and improve patient comfort. Material/Methods: Twenty patients with A-P LN metastases from a previous phase II study were included. For each patient, a planning CT (pCT) and 5 fraction CTs (fCT) were available. Based on the pCT, patients received a simulation plan of 5 x 9 Gy. The number of fractions was minimized isotoxically if the PTV coverage was ≥95% and the dose to the organs at risk (OAR) within constraints. Daily fractions were simulated on all 5 available fCTs with and without online adaptive radiotherapy (OART and N-OART, respectively). The plan was deemed deliverable if all target and OAR constraints were met. For patients with a favorable anatomy on an fCT, it was attempted to increase the fraction dose isotoxically to finish treatment earlier.

Figure 1: Schematic workflow with results

Results: Based on the pCT, the total number of fractions was reduced by 37% (p<0.01). 7 patients could be treated in 1 fraction and 3 patients in 2 fractions instead of 5. For 10 patients, a reduction was not possible. The plans were deliverable on the fCTs with OART in 97.1% of single-fraction cases and 53.3% of 2-fraction cases, compared to 34.4% and 20.0% without OART. For 4 patients (40%) with 5 fractions, in-treatment dose adaptation reduced the number of fractions by 16%, increasing the total reduction from 37% to 45%. OART significantly increased PTV D mean , D 98% and coverage, while decreasing OAR D max , D 0.5cc , D 1.0cc , D 2.0cc , and V 50Gy . In N-OART simulations, OAR constraint violations occurred in the 5- and 2-fraction groups, with D 0.5cc exceeded in 60% and 66.7% of patients. These violations were completely resolved using OART. Conclusion: Personalized fractionation with in-treatment dose adaptation reduced treatment fractions significantly by 45% when OART was used. This method increases treatment capacity, improves patient comfort, and provides significant PTV and OAR dosimetric benefits. A phase II trial is planned to assess the efficacy of this treatment method in a clinical setting.

Keywords: Oligometastases, OART, Hypofractionation