Highly conformal whole abdominopelvic radiotherapy: Consensus delineation guidelines from EpSSG and QUARTET

Purpose/Objective: Whole abdominopelvic radiotherapy (WAP-RT) is a complex treatment required for rare types of soft tissue sarcoma with peritoneal involvement. We present a consensus-based definition of the clinical target volume (CTVm_peritoneum) for WAP-RT for paediatric and adult patients. Material/Methods: QUARTET clinician reviewers for the Frontline and Relapsed Rhabdomyosarcoma (FaR-RMS) Trial (NCT04625907) met between March and September 2024 to review the WAP-RT plans submitted for radiotherapy quality assurance. Variations in CTVm_peritoneum delineation were identified. Consensus was informed by literature and reached following the Nominal Group Technique. Results: The most frequent variations in CTVm_peritoneum were at the cranial and caudal extent of the peritoneum, and the relation to adjacent organs at risk. The cranial limit is the diaphragmatic dome. A 4D-CT is recommended to identify the variation related to respiration, generating an internal target volume (ITVm_peritoneum). For cases without 4D-CT, the full respiratory excursion should be accounted for in the planning target volume (PTV) margins. The retroperitoneum should be excluded from the volume where not involved, using the iliac bifurcation as an anatomical surrogate landmark for the caudal extent. At 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 included entirely, while only the anterior component of the middle third is included. Both kidneys and the surrounding perirenal fat are excluded. The liver surface is included in the volume, encompassing the gallbladder, porta hepatis, hepatic hilum, and the falciform ligament. Conclusion: A consensus-based delineation guideline has been developed for CTVm/ITVm_peritoneum for WAP-RT. The atlas will be implemented to support clinicians for FaR-RMS, future QUARTET trials, and will be of benefit for other metastatic sarcomas with peritoneal involvement, such as desmoplastic small round-cell tumours.