S1010
Clinical – Paediatric tumours
ESTRO 2026
one. An internal target volume was submitted in 19 patients. Volumetric image guidance with CBCT was used in 416 patients: 71.6% daily, 14.7% weekly, 9.9% daily for the first 3–5 fractions, then weekly, and 3.8% on demand. Additional CT for offline replanning occurred in 29 patients, and MRI in 11. Planar imaging was reported for 265 patients, daily in 96.6% of cases.Offline replanning was reported in 30 cases (4
Hospitals NHS Foundation Trust, London, United Kingdom. 8 Département d'Oncologie Radiothérapie, Institut Universitaire du Cancer de Toulouse-Oncopole, Toulouse, France. 9 Department of Oncology, Oslo University Hospital, Oslo, Norway. 10 Department of Oncology, Rigshospitalet, University Hospital of Copenhagen, Copenhagen, Denmark. 11 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 12 Radiotherapy Department, The Oncology Institute “Prof. Dr. Ion Chiricuta”, Cluj- Napoca, Romania. 13 Institute of Advanced Studies in Science and Technology, Babe ș -Bolyai University, Cluj- Napoca, Romania. 14 Cancer Research UK Clinical Trials Unit (CRCTU), Institute of Cancer and Genomic Sciences, University of Birmingham, Birmingham, United Kingdom. 15 Children and Young People's Unit, Royal Marsden Hospital and Institute of Cancer Research, Sutton, United Kingdom. 16 Clinical Research Department, Institut Gustave Roussy, Villejuif, France. 17 Department of Pediatric and Adolescent Oncology, Institut Gustave Roussy, Villejuif, France. 18 Department of Radiation Oncology, Ghent University Hospital, Ghent, Belgium Purpose/Objective: Adaptive radiotherapy (ART) enables treatment modification in response to anatomical or motion changes, with potential improvements in target coverage and organ-at-risk (OAR) sparing. There is increasing interest in ART for paediatric treatments, yet application remains inconsistent. To assess the current practice and identify gaps, we performed an analysis on the usage of ART in two QUARTET-affiliated trials, which did not have any trial-specific ART requirements. Material/Methods: QUARTET Case Submission Forms and Facility Questionnaires for EpSSG-FaR-RMS (EUDRACT 2018- 000515-24) and SIOPEN-HR-NBL2 (EUDRACT 001068- 31) submitted before September 2025 were reviewed for any specified use of ART. Extracted variables included delivery technique, motion-management approaches, on-treatment imaging schedules, and documented replanning. Results: A total of 139 radiotherapy centres were approved using platforms such as Ethos (n=8), Tomotherapy units (n=28), and MRI-linacs (n=11). All other centres had C-arm linacs or proton beam delivery.Overall, 556 treatment plans were submitted: 350 photon, 205 proton, and 1 electron. The most used delivery techniques were VMAT (81.4%) and Tomotherapy (10.6%) for photons, and pencil-beam scanning (95.6%) for protons.Motion management with 4D-CT was documented in 92 patients (76% HR-NBL2; 24% FaR- RMS). In the FaR-RMS trial, breath-hold was recorded in 3 cases, and a controlled bladder-filling protocol in
photons and 26 protons). Reasons included anatomical changes (46.6%, figure 1), plan
optimisation (40%) and inter-fraction motion (13.3%), such as bladder filling or immobilisation issues. Five proton replans for anatomical changes were performed due to tissue density variation, including paranasal sinus filling (figure 2) or evolving seromas, consistent with the sensitivity of proton range to tissue heterogeneity. Where dosimetric triggers were specified (16 cases), reasons included dose to OARs (31.2%), target coverage (43.7%), hotspots (12.5%), or a combination of these (12.5%).
Conclusion: Adaptation-compatible workflows and imaging platforms for treatment adaptation are widely used within centres participating in QUARTET-affiliated trials. However, reported on-treatment imaging schedules and ART application remain heterogeneous. These findings establish the current baseline that can be used to support the implementation of more harmonised ART guidelines for paediatric radiotherapy. Keywords: Adaptive RT, rhabdomyosarcoma, neuroblastoma
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