S2756
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
Defining a tolerance threshold for bladder volume variability in CBCT-guided prostate VMAT: Toward optimized adaptive management Meriem Skouri, Zeineb Naimi, Raouia Ben Amor, Awatef Hamdoun, Roua Toumi, Raja Oueslati, Lotfi Kochbati Radiation Oncology Department, Abderrahmen Mami Hospital, Ariana, Tunisia Purpose/Objective: This study aimed to determine practical thresholds of bladder filling variability to guide on-treatment decision-making during CBCT-guided prostate radiotherapy. Material/Methods: Forty patients referred for definitive prostate cancer radiotherapy were prospectively enrolled. All patients were planned for VMAT with daily CBCT-guidance and a prescription dose of 76 Gy in 38 fractions. To assess bladder volume variability, the bladder was delineated on all CBCT scans (n=1520). The initial treatment plan was recalculated on each CBCT, to obtain delivered bladder dose-volume parameters. Relative bladder volume variability was defined as the mean percentage deviation from the planning bladder volume across all treatment sessions, and relative dosimetric variability as the mean percentage deviation from the planned dose. Pearson correlation and linear regression analysis were used to assess the association between volumetric and dosimetric variabilities. ROC analysis was conducted to determine thresholds of bladder volume variability predictive of a significant dosimetric impact, defined as a relative dosimetric variability exceeding 20%, 30% or 50% in at least one bladder dose-volume parameter (V50-75Gy). Results: Mean bladder volume during treatment was 224.1 ml versus 344.8 ml at planification, with a relative bladder volume variability of -30.9%. In parallel, all bladder dose-volume parameters significantly increased during treatment (p<0.001). Bladder volume reductions correlated significantly and linearly with bladder dose increases (p<0.001). Volumetric variability thresholds of -27%, -31% and -33% were predictive of dosimetric deviations of >20%, >30% et >50%, respectively (AUC range: 0.77 – 0.85), with an average cut-off around -30%.
prostate radiotherapy. Incorporating this threshold in daily practice may guide adaptive treatment decisions, improve workflow efficiency, and enhance patient safety and comfort. Keywords: Prostate cancer, Bladder variability, Threshold
Proffered Paper 5131
Pediatric VMAT-TBI in allogeneic HSCT: Dosimetric fidelity and reproducibility under CBCT guidance with Systematic In Vivo Dosimetry Sara Iacovone 1 , Elisa Meldolesi 2 , Antonio Matteo Perfido 1 , Francesca Greco 3 , Gabriele Turco 1 , Patrizia Cornacchione 1 , Silvia Mariani 1 , Gerardina Stimato 4 , Stefano Mazzi 1 , Flavia De Giacomo 1 , Elisa Marconi 5 , Lucrezia Bernabucci 1 , Veronica Pollutri 1 , Marco Bianchi 1 , Patrizia Ranaldi 1 , Maria Serpone 1 , Nicola Dinapoli 1 , Vincenzo Frascino 1 , Marco De Spirito 3 , Maria Antonietta Gambacorta 2 , Silvia Chiesa 2 1 Dipartimento di Diagnostica per Immagini, Fondazione Policlinico Universitario A. Gemelli IRCCS, Rome, Italy. 2 Radiation Oncology Unit, Diagnostic Imaging and Radiation Oncology Department, Fondazione Policlinico Gemelli IRCCS, Rome, Italy. 3 Department of Radiology and Oncological Radiotherapy, UOC Fisica per le Scienze della vita, Fondazione Policlinico Gemelli IRCCS, Rome, Italy. 4 Department of Radiology and Oncological Radiotherapy, UOC Fisica per le Scienze della vita, Fondazione Policlinico Universitario Agostino Gemelli IRCCS, Rome, Italy. 5 Clinical Psychology Unit, Fondazione Policlinico A. Gemelli IRCCS, Rome, Italy Purpose/Objective: To evaluate the dosimetric accuracy, setup reproducibility, and robustness of VMAT-TBI in pediatric patients undergoing myeloablative conditioning for allogeneic HSCT. The study included systematic in-vivo dosimetry (IVD), CBCT-based verification, and analysis of treatment variables, with dedicated assessment of a modified regimen for patients treated under anesthesia. Material/Methods: VMAT–TBI plans were generated in Eclipse (v18.1). Delivery used 7–9 partially overlapping isocenters, optimized to provide contiguous whole-body coverage while meeting OAR constraints. The standard conditioning regimen was 12 Gy in 6–8 fractions delivered twice daily. For patients requiring anesthesia, a once-daily hypofractionated schedule totaling 9.99 Gy in 3 fractions was adopted to accommodate clinical and logistical considerations. Image guidance consisted of cone-beam CT (CBCT) at every isocenter for every fraction to quantify
Conclusion: To our knowledge, this is the first study to define a tolerance threshold for bladder volume variability in
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