S2151
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Conclusion: 3D EPID-IVD, combined with SGRT, effectively detects patient-specific delivery deviations and supports IGRT protocol selection. Scheduled IGRT was more frequently associated with suboptimal setups and increased EPID-IVD deviations. These findings support integrating EPID-IVD and SGRT as complementary tools for continuous quality assurance and optimization of breast radiotherapy protocols. References: [1] M.E. Mast, A. Leong, S.S. Korreman, G. Lee, H. Probst, P. Scherer, Y. Tsang, ESTRO-ACROP guideline for positioning, immobilisation and setup verification for local and loco-regional photon breast cancer irradiation, Technical Innovations & Patient Support in Radiation Oncology,2023, https://doi.org/10.1016/j.tipsro.2023.100219. Keywords: EPID in vivo dose, IGRT protocol, breast cancer In vivo dosimetry with flat panel detectors as a driver for offline adaptive radiotherapy and immobilisation improvement Elena González González, Patricia López-Blanco Díaz, Héctor Gómez Pérez, Sheila Calvo Carrillo, Alejandro Barranco López, Beatriz García Costa, Sonia Serrano Zabaleta, Raquel Castro Moreno, Celia Castán Guerrero, Francisco Javier Jiménez Albericio, Pablo Ortega Pardina, Pedro Ruiz Manzano, Ana Millan Armengol, Alejandro García Romero Physics and Radiation Protection, Hospital Clínico Universitario Lozano Blesa, Zaragoza, Spain Digital Poster 3237 Purpose/Objective: With the increasing complexity of external beam radiotherapy (EBRT), verifying that the dose delivered to the patient matches the planned dose has become essential. In vivo dosimetry using the treatment beam enables near real-time assessment of delivered dose distributions, allowing early detection of deviations in patient positioning, anatomy, or set-up that could compromise treatment quality. This study aims to evaluate the clinical value of flat-panel in vivo dosimetry as both a quality assurance tool and a trigger for offline adaptive radiotherapy. Material/Methods: During EBRT for selected sites (head and neck, lung, prostate, and rectum), megavoltage (MV) images were acquired using the on-board flat-panel detector of a TrueBeam linear accelerator (Varian Medical Systems). These images were automatically processed by the PerFraction software (Sun Nuclear), which compares predicted and measured dose maps for each treatment arc. For head and neck patients, verification
and last fraction). Agreement between treatment planning system (TPS) and EPID-IVD was evaluated using gamma passing rate and PTV-DVH endpoints. SGRT data (VisionRT, UK) were collected to derive translational and rotational beam-on displacement metrics, representing combined residual inter- (with CBCT) or total inter- (without CBCT) and intra-fraction errors. Patients were classified as WELL or BAD setups using thresholds of 2.5 mm and 0.5°. Correlations between EPID-IVD, SGRT indices, and IGRT protocol were tested using non-parametric statistics. Results: No significant differences were observed between fractionation schemes in EPID-IVD (p = 0.1547). Significant variations were found between IGRT protocols: scheduled IGRT showed lower GPR% (p < 0.0001) and higher PTV-DVH deviations for D%(V95%) (p = 0.0103) (Figure 1). EPID-IVD deviations correlated with SGRT classifications, with BAD setups showing lower GPR and larger PTV dose errors, more frequently under scheduled IGRT (Figure 2). Paired comparisons in scheduled IGRT patients between fractions with and without CBCT confirmed a median GPR% decrease from 82.7% to 74.6% (p = 0.0127) and an increase in DVH deviations from 3.3% to 4.6% (p = 0.0203) and from 1.8% to 3.2% (p = 0.0032) for D%(V95%) and D%(50%), respectively.
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