S2029
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Radiology and Oncological Radiotherapy, UOC Fisica per le Scienze della vita, Fondazione Policlinico Gemelli IRCCS, Rome, Italy. 3 Clinical Psychology Unit, Fondazione Policlinico Gemelli IRCCS, Rome, Italy Purpose/Objective: The objective of this study was to quantify inter- isocenter setup displacements and rotations across anatomical regions during pediatric VMAT-TBI and to estimate the optimal CTV–PTV margins using the van Herk formula. The analyzed axes included lateral (X), longitudinal (Y), and vertical (Z) directions. Material/Methods: VMAT-TBI plans were generated from two CT datasets—head-first (HF) and feet-first (FF). HF employed multi-isocenter 6-MV VMAT arcs, FF static AP/PA fields for the lower limbs. Per-fraction couch corrections were recorded at head, upper thorax, lower thorax, lumbar region, pelvis, femur, feet levels. For each region and axis, we computed the mean shift (systematic component, Σ ) and the standard deviation (random component, σ ). Patients were immobilized with a cranial thermoplastic mask and a full-body vacuum cushion on a rotatable tabletop. Optimal CTV– PTV margins were derived with the van Herk formula (2.5 Σ + 0.7 σ ) to ensure that patients receive at least 95% of the prescribed dose. Results: Head and upper thorax regions showed near-zero mean shifts and low variability, confirming highly reproducible positioning. Lower trunk exhibited small systematic tendencies, whereas distal segments, particularly femur and feet, displayed the largest variability, predominantly along the Y axis (Figure 1). Rotational offsets were minimal at all levels and below 1.3°. Margin estimates followed a cranio-caudal gradient. Head and upper thorax required approximately 0.5–1 mm across axes. Lumbar and pelvic regions showed up to about 2.8 mm along Y. Femoral sites were nearly isotropic around 2.5 mm, while the feet demanded the largest longitudinal expansion, with optimal margins approaching 6.6 mm. Graphical summary (Figure 2) underscores this progressive increase from proximal to distal sites and highlights the value of reinforced lower-body immobilization to limit longitudinal displacement.
dose remained < 1 Gy. Beam-on time decreased by 20 ± 2.7 s (p < 0.01). All OAR constraints were met.
Figure A. Axial isodose distribution of single-arc hybrid VMAT-SIB (26Gy/5 fx to whole breast with boost) in left-sided breast cancer, showing high conformity and minimal low-dose spill to ipsilateral lung and heart.Figure B. Corresponding full-arc VMAT-SIB plan demonstrating increased lung V5 Gy. Conclusion: A single-arc hybrid VMAT-SIB achieved excellent target coverage with a significant reduction in cardiac and pulmonary dose, while improving delivery efficiency compared to full-arc VMAT. The technique is simple, reproducible, and feasible for routine implementation in hypofractionated breast radiotherapy. A prospective evaluation of late toxicity and long-term local control is necessary. References: [1] Darby S, et al. Lancet 2011;378:1707-16. doi:10.1016/S0140-6736(11)61629-2[2] Lohr F, et al. Int J Radiat Oncol Biol Phys 2009;74:73-80. doi:10.1016/j.ijrobp.2008.07.018[3] Franceschini D, et al. Radiother Oncol 2021;164:50-6. doi:10.1016/j.radonc.2021.09.006[4] Ashby O, Bridge P. Radiography 2021;27:650-3. doi:10.1016/j.radi.2020.08.003[5] Kendall R, et al. Adv Radiat Oncol 2025;10:101016. DOI:10.1016/j.adro.2025.101728[6] Cilla S, et al. Strahlenther Onkol 2022;198:254-67. doi:10.1007/s00066-021-01873-3[7] Boccardi M, et al. Breast Cancer Targets Ther 2024;16:611-9. doi:10.2147/BCTT.S470417[8] Choi SH, et al. Adv Radiat Oncol 2025;10:101733. doi:10.1016/j.adro.2025.101733 Keywords: Single arc planning, OAR sparing Digital Poster 5119 From head to feet: spatial analysis of setup displacements, optimal margins estimation and practical solutions in pediatric VMAT-TBI Elisa Meldolesi 1 , Silvia Chiesa 1 , Sara Iacovone 1 , Silvia Mariani 1 , Stefania Teodoli 2 , Gerardina Stimato 2 , Vincenzo Frascino 1 , Flavia De Giacomo 1 , Mariangela Massaccesi 1 , Gabriele Turco 1 , Elisa Marconi 3 , Antonio Matteo Perfido 1 , Marco De Spirito 2 , Maria Antonietta Gambacorta 1 , Francesca Greco 2 1 Radiation Oncology Unit, Diagnostic Imaging and Radiation Oncology Department, Fondazione Policlinico Gemelli IRCCS, Rome, Italy. 2 Department of
Figure 1.
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