S1710
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
Luisa Gómez Chapel, ángela Fonseca Siles, Maria Fustes Sanjuán, Isabel Castro Rizos, Samuel Díaz del Álamo Sánchez Crespo, Carla Sanchez Cortes, Sara Sastre Gallego Radiotherapy, University Hosptial Toledo, Toledo, Spain Purpose/Objective: Treatment interruptions in radiotherapy are known to negatively impact tumour control, particularly in fast- proliferating tumours. Our department had already implemented Saturday treatments to mitigate delays; however, a structured, evidence-based system was developed to optimize management, based on the Royal College of Radiologists (RCR, 4th ed.) and the forthcoming Spanish Society of Medical Physics (SEFM) guideline on treatment interruption management (presented at the SEFM 2024 Annual Congress). This study describes the implementation process, ethical framework, and first results in Category 1 and Category 2 treatments Material/Methods: According to RCR and SEFM classification, Category 1 includes fast-growing tumours (e.g., head and neck, lung, cervical), where interruptions >2 days may impact tumour control. Category 2 includes slower- growing tumours (e.g., breast, prostate) where interruptions >5 days may have a moderate effect. A multidisciplinary team reviewed recommendations and defined a local protocol incorporating SEFM ethical principles (beneficence, non-maleficence, justice, autonomy) and feasibility criteria. A multidisciplinary group (radiation oncologist, medical physicist, and scheduling technician) now meets weekly every Wednesday to review ongoing cases and decide which patients will be scheduled for Saturday sessions. The protocol prioritizes maintaining overall treatment time (OTT). An Excel-based tool was created to register interruptions, classify them, and guide compensatory actions. Data were collected for 2024 (pre-implementation) and prospectively since end of July 2025 (post-implementation). Results: After protocol activation (July–October 2025) interruptions decreased notably: in Category 1, the mean proportion of patients with interruptions longer than 2 days dropped from 8% to 2%, and in Category 2, interruptions longer than 5 days decreased from 7% to below 1%. A marked increase in interruptions for both Cat.1 and Cat.2 was observed during public holidays (April, May, Easter), consistent with prior publications on the seasonal effect of non-working days. Saturday sessions are pre-planned, no increase in staff workload was observed. Figures 1 and 2 summarize the monthly evolution of interruptions for Categories 1 and 2.
Fig. 2. Beamlet overlaid on CT and range estimates averaged with respect to sonar channel. Results: For channels 1-4, range estimates were all positive and agreed to within 800 μ m (800 μ m to 1.6 mm). Precision (standard deviation) was worse, ranging from 900 μ m to 2.3 mm. Channels 5-6 yielded unacceptable results, due to poor SNR of measured signals and inaccurate simulations through ribs and around air. Conclusion: Range estimates are extremely sensitive to co- registration errors, and could be improved by leveraging optical accuracy of surface guidance systems. Ionoacoustic range verification has clinical promise, although signal is low during conventional dose rate delivery. Increased SNR–and therefore– precision could be achieved by hypofractionation (reduce noise) and FLASH (increase signal). Finally, ultrasound imaging provides excellent soft-tissue contrast at frame rates exceeding 10 Hz. References: 1. Hao Y, Mai T, Green W, McElvain K, Traneus E, Patch S. Thermoacoustic range verification during conventional dose rate delivery by a synchrocyclotron. Medical Physics. 2025;52(7):e17954. doi:10.1002/mp.17954 Keywords: range verification, ionoacoustic, ultrasound Implementation of an organized management system for radiotherapy treatment interruptions: first results following SEFM and RCR recommendations Clara Navarro Ibarra, Miguel Angel Díez Gallego, Francisco Clemente Gutiérrez, Maria Paz García de Acilu Laa, Eduardo Pardo Pérez, Noelia Solís Preciado, José Ángel Merino Gestoso, Angel Forner Forner, Maria Digital Poster 987
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