S1319
Clinical - Urology
ESTRO 2026
atlas-based segmentation and reviewed by an experienced radiation oncologist. Deformable image registration was performed between fractioned and reference images, with adaptation to patient position and validation through quality assurance. The dose of the treatment day was transferred to the reference MR image (1st fraction MR) and then summed to estimate the dose of the treatment course (Figure 1). Dose differences between the accumulated dose and the initial planned dose were evaluated. Dosimetric parameters were extracted from dose-volume histogram. The difference between grade <2 and ≥ 2 of genitourinary and gastrointestinal side-effect events and dosimetric was identified.
577. doi:10.1093/annonc/mdt433.2. Hanlon JT, Schmader KE. The Medication Appropriateness Index: a clinimetric measure. Psychother Psychosom.2022;91(2):78-83. doi:10.1159/000521699.3. U.S. DHHS, NIH, NCI. Common Terminology Criteria for Adverse Events (CTCAE) v5.0. Nov 27 2017. Keywords: Prostate cance, Polypharmacy, Acute toxicity Digital Poster 5157 Dose Accumulation Analysis and Side Effects Relationship in MR-guided Adaptive Radiotherapy for Prostate Cancer Sumetha Fang 1 , Tanwiwat Jaikuna 1 , Tissana Prasartseree 1 , Kongpop Chukly 1 , Naphat Komenake 1 , Akrapol Suppasedtanon 1 , Thee Mateepithaktham 1 , Wiwatchai Sittiwong 2 , Pongpop Tuntapakul 1 , Wajana Thaweerat 1 , Pitchayut Wongsuwan 1 , Ratchapas Romrattaphan 1 , Marianne Aznar 3 , Eliana Vasquez Osorio 3 1 Division of Radiation Oncology, Department of Radiology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand, Bangkok, Thailand. 2 Department of Radiation Oncology, University of Texas MD Anderson Cancer Center, Houston, TX, USA. 3 Division of Cancer Sciences, School of Medical Sciences, Faculty of Biology, Medicine and Health, The University of Manchester, Christie NHS Foundation Trust Hospital, Manchester, United Kingdom Purpose/Objective: MR-guided online adaptive radiotherapy (MRgART) enables real-time treatment adaptation based on daily anatomy, potentially reducing radiation-induced side effects and maintaining target dose coverage. According to the changing of dose distribution and organ deformation fraction-to-fraction, accurate dose accumulation across treatment fractions is essential for estimating the expected dose in the patient throughout the treatment course. This study aims to estimate spatial dose accumulation throughout the entire course of prostate stereotactic body radiotherapy (SBRT) using deformable image registration and compare the benefits of online adaptive radiotherapy, considering the relationship between side-effect event dose and accumulated dose, as well as between the initial planning dose and accumulated dose. Material/Methods: Forty-six prostate cancer patients treated with 5 fractions of 7.25 Gy on a 1.5 T MR-Linac were included (where the treatment is adapted based on patient positioning changes between fractions). Organs on daily MR images were automatically segmented using
Results: With online adaptive MRgART, target dose coverage remains similar. However, after normalising the dose to the delivery position and shape of normal organs, doses tend to be higher in the accumulated dose compared to the initial planned doses at D2% of the bladder. Dose-side effects event analysis revealed that a higher dose to the bladder generally correlates with increased genitourinary side-effects grades with MRgART. Each dose parameter showed varying degrees of correlation with toxicity outcomes, with dose-volume metrics demonstrating different levels of association, particularly regarding genitourinary complications. Conclusion: Dose accumulation in online MRgART differs from the initially planned dose and tends to show stronger correlations with side effects than the original plan. This study emphasises the importance of reporting the estimated dose accumulation throughout the treatment course and utilising the accumulated dose as a factor for predicting side effects after
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