ESTRO 2025 - Abstract Book

S1646

Clinical – äediatric tumours

ESTRO 2025

References: 1. Davey et al., The need for consensus on delineation and dose constraints of dentofacial structures in paediatric radiotherapy: Outcomes of a SIOP Europe survey,Clin Transl Radiat Oncol, 2023. doi:10.1016/j.ctro.2023.100681.

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Poster Discussion A retrospective observational study of 3D set-up error evaluation in children on radiotherapy treatment of extra- cranial paediatric solid tumours. Rishika R Mody, Nehal R Khanna, Siddharth Laskar, Jifmi J Manjali Radiation Oncology, Tata Memorial Centre, Mumbai, India Purpose/Objective: This study aims to assess three-dimensional (3D) set-up errors in paediatric radiotherapy for solid tumours and recommend optimal target volume margins. The primary objective is to evaluate set-up errors in children treated at our centre, while secondary objectives include comparing set-up errors by anatomical site (head and neck, thorax, abdomen and pelvis, extremities), assessing the impact of anaesthesia and age on set-up accuracy, and proposing optimal CTV-PTV margins to improve target coverage and minimize normal tissue irradiation. Material/Methods: This retrospective study included 130 paediatric patients undergoing radiotherapy on a Linear Accelerator (LA) with CBCT. Patients were immobilized using thermoplastic masks specific to the treatment site (head and neck, thorax, abdomen-pelvis, or extremities). Set-up errors were assessed using kVCT imaging and 3D translational measurements (lateral, vertical, and longitudinal). PTV margins were calculated using Van Herk and Stroom formulas. Image-guided radiotherapy (IGRT) was performed with offline and online correction protocols to minimize errors. A subset analysis evaluated the impact of age, treatment site, and anaesthesia on set-up accuracy.

Results:

The study analysed 1543 CBCT images from 130 paediatric patients, assessing displacements in the AP, SI, and ML directions. 97% of displacements were within 5 mm, with the largest displacement in the craniocaudal direction, with greater inaccuracies noted from the head and neck to the pelvic region due to motion variability. Based on systematic and random error data, optimal CTV-PTV margins are recommended: 5 mm for head/neck, 8-10 mm for thoracic, and 10-12 mm for abdominal/pelvic regions, ensuring proper target coverage while minimizing under- and over-coverage. Anaesthesia did not significantly affect displacement, and age had minimal impact on errors. The study highlights the need for personalized set-up margins and emphasizes the importance of tailored immobilization and frequent imaging.