ESTRO 2025 - Abstract Book

S3155

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Conclusion: The results suggest that two-dimensional kV imaging is comparable to CBCT when used with four-dimensional patient support. Furthermore, proper immobilization plays a critical role in minimizing setup errors, regardless of the IGRT modality employed.

Keywords: Radiotherapy, Head and Neck cancer, Brain cancer

2861

Digital Poster Implementation of an in vivo dosimetry programme for paediatric VMAT Total Body Irradiation Matteo Galetto, Gerardina Stimato, Stefania Teodoli, Cintia De Almeida Ribeiro, Elisa Meldolesi, Silvia Chiesa, Marco De Spirito, Maria Antonietta Gambacorta, Francesca Greco Dipartimento di diagnostica per Immagini e Radioterapia Oncologica, Fondazione Policlinico Universitario Agostino Gemelli IRCCS, Roma, Italy Purpose/Objective: VMAT Total Body Irradiation (TBI) treatments are gaining popularity because they allow for better modulation of the dose to organs at risk (OAR), compared to traditional extended SSD 3DCR treatments 1,2 . In vivo dosimetry (IVD) methods can be employed to evaluate the effectiveness of the treatment. Material/Methods: Eight TBI for paediatric patients were realized with a multiple isocenter VMAT treatment plan with a dose prescription of 12Gy/6fx twice a day or 9.99Gy/3fx single fraction per day in case of anaesthesia. Patients were immobilized using thermoplastic mask and vacuum bed over a rotatable tabletop. The patient is positioned supine on the couch in head-first setup and the number of VMAT isocenters depends on patients’ dimensions. After the erogation of the VMAT isocenters, the tabletop is rotated, and legs are treated in feet-first setup using square open fields in AP-PA. Treatment planning, image registration and dose summation were performed using the Varian Eclipse (v.18.1) TPS. During every treatment fraction a CBCT is performed for each isocenter for setup verification and IVD were acquired with the EPID for every arc. The IVD analysis is performed with the software SunCheck PerFRACTION with a gamma analysis (global, 7%-3mm). An offline review of the CBCT is then compared with IVD results. Results: The mean Gamma Passing Rate (GPR) of IVD results of all isocenter per each patient ranges from 93.6% to 99.3%, GPR ranges between 74.4% and 100% for all the single arcs of the different patients (Figure). Out of the 412 measured IVD, 26 (6.3%) show a GPR below 95% and 24 of them are from the pelvic area isocenters. By specifically analysing the CBCT of the 26 IVD with a GPR<95% the main sources of errors were found to be incorrect positioning of the patient's arms, patient rotations, differences in bladder and gastro-intestinal organs filling.