ESTRO 2025 - Abstract Book

S1618

Clinical – äediatric tumours

ESTRO 2025

977

Proffered Paper EpSSG-QUARTET consensus-based delineation guideline for whole abdominopelvic radiotherapy Maria Chiara Lo Greco 1,2 , Sarah M. Kelly 1,2,3 , Coreen Corning 2 , Raquel Dávila Fajardo 4 , Henriette Magelssen 5 , Alison Cameron 6 , Mónica Ramos Albiac 7 , Valérie Bernier-Chastagner 8 , Giovanni Scarzello 9 , Akmal Safwat 10 , Mark N. Gaze 11 , Tom Boterberg 12 , Amos Burke 13 , Julia Chisholm 14 , Henry C. Mandeville 1,14 1 Quality and Excellence in Radiotherapy and Imaging for Children and Adolescents with Cancer across Europe in Clinical Trials (QUARTET), European Society for Paediatric Oncology (SIOP Europe), Brussels, Belgium. 2 Medical Department, Radiotherapy Quality Assurance (RTQA) unit, European Organisation for Research and Treatment of Cancer (EORTC), Brussels, Belgium. 3 Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium. 4 Department of Radiation Oncology, University Medical Center, Utrecht, Netherlands. 5 Department of Oncology, Oslo University Hospital, Oslo, Norway. 6 Bristol Haematology and Oncology Centre, University Hospitals Bristol and Weston NHS Foundation Trust, Bristol, United Kingdom. 7 Radiation Oncology Department, Hospital Universitario Valle de Hebrón, Barcelona, Spain. 8 Department of Radiation Oncology, University Regional Hospital Center, Nancy, France. 9 Radiotherapy Unit, Istituto Oncologico Veneto, Padua, Italy. 10 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 11 Department of Oncology, University College London Hospitals NHS Foundation Trust, London, United Kingdom. 12 Department of Radiation Oncology, Ghent University Hospital, Ghent, Belgium. 13 Cancer Research UK Clinical Trials Unit (CRCTU), Institute of Cancer and Genomic Sciences - Sponsor representative, University of Birmingham, Birmingham, United Kingdom. 14 Children and Young People's Unit, Royal Marsden and Institute of Cancer Research, Sutton, United Kingdom Purpose/Objective: Whole abdominopelvic radiotherapy (WAP-RT) is a complex treatment required for rare types of paediatric soft tissue sarcoma with peritoneal involvement. We have undertaken to produce a consensus-based definition of the clinical target volume (CTVm_peritoneum) for WAP-RT in patients with soft tissue sarcoma. Material/Methods: QUARTET clinician reviewers for the Frontline and Relapsed Rhabdomyosarcoma (FAR-RMS) Trial (NCT04625907) from the EpSSG Radiotherapy Committee met between March and September 2024 to review all patients treated with WAP-RT, enrolled in FAR-RMS. Common sources of variations in CTV delineation of the entire peritoneal cavity were identified. Consensus was reached using an iterative approach and the development of test contouring cases. Results: Common sources of CTV variations were the cranial (figure 1 a) and caudal (figure 1 b) extent of the peritoneum, and the relation to adjacent organs at risk (OARs) (figure 1 c). Cranial limit is defined as the diaphragmatic dome using a 4D-CT to identify the maximum inspiration point. For cases without 4D-CT, the full respiratory excursion should be incorporated in the PTV margins. The iliac bifurcation can be used as an anatomical landmark for the caudal extent of retroperitoneum [1]. Para-aortic nodal stations should be delineated separately for nodal involvement. For the caudal limit of the peritoneum, the bladder should be entirely included, unless there is controlled bladder filling, or adaptive protocols, when the bladder can be gradually excluded from the superior aspect of the symphysis pubis. The upper third of the rectum is to be included entirely, but only the anterior component of the middle third, ensuring coverage of the space between the anterior rectal wall and the bladder, prostate or uterus. For OARs, both kidneys and surrounding perirenal fat (if visible) can be excluded. The liver surface is to be included into the volume (together with the gallbladder, porta hepatis, hepatic hilum, and the falciform ligament, if visible). Ideally modifications around the liver and kidneys, should be informed by 4D-CT to account for organ motion.