S1681
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
1 Unit of Medical Physics, ASST Santi Paolo e Carlo, Milan, Italy. 2 Unit of Medical Physics, European Institute of Oncology (IEO) IRCCS, Milan, Italy Purpose/Objective: Protontherapy applications such as ocular tumors and stereotactic treatments can require fields with sharp lateral penumbras and robust small-field dosimetry. This study presents a preliminary characterization of small-field pencil-beam scanning (PBS) delivery for protontherapy, combining point and planar detectors and investigating the impact of aperture-based beam shaping and nozzle configuration parameters. Material/Methods: Several detector technologies were investigated: PTW microDiamond (mD), Blue Physics Model 11 plastic scintillation detector (BP-PSD) characterized for the first time in proton beams, IBA PPC05 ionization chamber, IBA Lynx-PT and myQA-Phoenix 2D systems. Measurements included percentage-depth-dose (PDD) curves, lateral profiles, output factors (OFs) for field sizes down to 0.5cm fields at 70MeV. Additional studies assessed spot-spacing effects and the influence of brass apertures, range-shifter (RS) positioning, and air-gap variations on the 80%–20% lateral penumbra and effective field size. A summary of all measurements and detector configurations is reported in Table1 Results: The mD accurately reproduced TPS ranges and distal fall-offs, showing a modest (~5%) over-response at the Bragg peak for low energies. Lateral profiles agreed with TPS and 2D detectors, with a slight tail over- response attributable to sensitivity to low-energy secondaries. The BP-PSD showed linear response at 70 and 200MeV with millisecond-scale temporal resolution sufficient to resolve the PBS burst structure; however, after high-MU irradiations at 200MeV a progressive signal decrease was observed. Quenching effects produced an under-response near the Bragg peak, which was corrected using a simple linear quenching model, restoring PDD agreement with mD and TPS data. For OFs (70MeV, 0.5-10cm), BP-PSD and mD yielded consistent results, whereas the PPC05 ionization chamber measures systematically lower for the smallest fields, consistent with expected volume- averaging effects. Regarding spot-spacing, all measured values exhibit the same behavior, with the signal decreases as the spot spacing increases. Planar- detector benchmarking showed excellent agreement between myQA-Phoenix, Lynx-PT, and TPS for open
Conclusion: This study demonstrates the feasibility of integrating a scintillation-based 2D dosimetry system into a breathing anthropomorphic phantom for real-time verification of adaptive lung radiotherapy. The system’s high temporal resolution and strong correlation with planned dose maps make it a robust candidate for on-the-fly adaptive QA and future AI- driven beam control frameworks in clinical environments. References: Nascimento, L. F., Verellen, D., Goossens, J., Struelens, L., Vanhavere, F., Leblans, P., & Akselrod, M. (2020). Two-dimensional real-time quality assurance dosimetry system using μ-Al2O3: C, Mg radioluminescence films. Physics and imaging in radiation oncology, 16, 26-32.Fonseca, G. P., Rezaeifar, B., Lackner, N., Haanen, B., Reniers, B., & Verhaegen, F. (2023). Dual-energy CT evaluation of 3D printed materials for radiotherapy applications. Physics in Medicine & Biology, 68(3), 035005.VERIFIED Consortium, PIANOFORTE2023-027 Project Report, 2024. Keywords: Scintillator, 2D dosimetry, real time Digital Poster Highlight 4180 Preliminary characterization of small-field pencil beam scanning in proton therapy Valerio Ricciardi 1,2 , Floriana Pansini 2 , Marco Liotta 2 , Stefania Comi 2 , Federica Cattani 2
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