S1769
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
behind the phantom recorded the transmitted signal. The calculated 2D nominal dose distribution at the detector plane was compared with measurements in the reference configuration and under ±1 mm and ±2 mm cranio-caudal (CC) isocenter shifts to assess on- line verification sensitivity. To test clinical feasibility, 10 skull-base plans (55.8 GyE/31fx) optimized with 10 TBs were compared to gantry-based PT plans (4 beams) achieving identical target coverage and brain OAR constraints (figure 2).Robustness was evaluated under both a standard clinical scenario (2 mm/±3.5%) and an additional extreme range uncertainty scenario (±8%). Results: FS after the slab phantom increased by 0.5%, 0.9%, and 3.2% after 7, 17, and 27 cm in air, respectively, compared to the phantom exit. Correspondingly, LP increased by 28%, 66%, and 121%. In the head phantom experiment, gamma pass rates (1.5mm/2%) between nominal and measured 2D dose distributions were 91.6%, 90.3%, and 89.8% for 0, +1mm, and +2 mm CC shifts, respectively. In the in-silico study, TB
on cCBCT, was 99.72% (global-gamma 2%/2mm). In AD plans, although variation in daily filling and position for bladder and rectum were observed, CTV coverage and OAR clinical goals remained within institutional limits. The GPR between PRP and AD was 99.7 ± 0.1% (global 2%/2 mm), confirming robust dose consistency. Conclusion: The cCBCT algorithm is a reliable tool for offline APT in prostate treatments, for plan recalculation and dose accumulation throughout the treatment course, ensuring appropriate patient treatment and possibly becoming part of the clinical workflow. Our analysis suggests that cCBCT may reduce verification CTs in hypofractionated or normofractionated PT, with re- planning needed only when clinical constraints are exceeded. Keywords: adaptive, particle therapy, CBCT Transmission proton therapy for skull-base tumors: on-line positional verification and enhanced robustness Alessandro Vai 1 , Maria Plana Herranz 1,2 , Alfredo Mirandola 1 , Sara Lillo 3,4 , Giuseppe Magro 1 , Alessia Bazani 1 , Silvia Molinelli 1 , Eleonora Rossi 1 , Stefania Russo 1 , Alberto Iannalfi 3 , Luca D'Ambrosio 3 , Erik Engwall 5 , Mario Ciocca 1 , Ester Orlandi 3,6 1 Medical Physics, CNAO, Pavia, Italy. 2 Physics, University of Pavia, Pavia, Italy. 3 Radiotherapy, CNAO, Pavia, Italy. 4 Internal Medicine and Therapeutics, University of Pavia, Pavia, Italy. 5 Research and Development, RaySearch Laboratories, Stockholm, Sweden. 6 of Clinical, surgical, Diagnostic and Pediatric Sciences, University of Pavia, Pavia, Italy Purpose/Objective: The aim of this study was to investigate the use of proton therapy (PT) transmission (or shoot-through) beams for skull-base tumors treatments, focusing on on-line positional verification and plan robustness, prospecting an upright treatment setup. Material/Methods: Digital Poster 4178 To experimentally evaluate the scattering of a proton field after traversing a thick target, a single square (3.5 × 3.5 cm ² ) monoenergetic proton field (320 mm range in water) was delivered to a 15-cm-thick RW3 slab phantom. Six EBT3 films were placed within and beyond the phantom in air to measure field size (FS) and lateral penumbra (LP) with respect to the beam exit. Subsequently, a pseudo-clinical plan optimized with the same energy in transmission (TB) on an anthropomorphic head phantom simulating a skull- base tumor adjacent to the brainstem was delivered with an upright setup (figure 1). A commercial scintillation detector (Lynx, IBA) positioned 11.7 cm
plans met identical target coverage and OAR constraints as gantry plans. Robustness was
enhanced, with dose differences between nominal and worst-case scenarios <1.0% (2mm/3.5% and 8%) on average, compared to >5.0% (2mm/3.5%) and 20.0% (8%) in gantry plans. However, TB plans showed a larger low-dose bath in surrounding healthy tissues, e.g., Brain_Dmean (+2.5 GyE), TemporalLobeomolateralDmean(+2.6 Gy) and TemporalLobecontralateralDmeancontralateral (+10.2 Gy) .
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