S2133
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Gy). For both parameters, MIM-Rigid and MIM-DIR- Image predicted higher accumulated doses. Despite these differences, the decisions on in-treatment dose adaptation of TPS-RPM and DVH summation were identical: feasible for 9/25 patients (36%), reducing total fractions by 17%, similar to previous findings (16%) [1].
Schiller-University Jena, Jena University Hospital, Jena, Germany
Purpose/Objective: Neoadjuvant pelvic radiotherapy is applied in locally advanced rectal cancer (LARC) to reduce local recurrence rates after radical resection.(1) Increasing the radiation dose is presumed to improve complete response rates, enabling organ-preserving approaches.(2) However, interfractional variations, mainly due to motion and variability in organ filling (3), need to be compensated during fractionated external beam radiotherapy. Data on the extent and safety margins to compensate for interfractional motion are limited. This study aims to investigate these effects systematically within a prospective study framework. Material/Methods: We analyzed 80 consecutive T2-weighted MRI scans from 20 patients with LARC undergoing neoadjuvant CRT. Images were collected within the prospective PRIMO study (NCT05524012) on a 3T scanner following the study imaging protocol before treatment (T1), at week 3 (T2) and week 5 (T3) during CRT, and week 8 (T4). Radiotherapy-specific immobilization devices were used to obtain reproducible treatment positions (supine, head first). A gross tumor volume (GTV) was contoured manually on axial slices, consisting of tumor-related tissue before and during treatment, including either tumor mass or fibrotic scar tissue. After rigid fusion, established motion parameters for the GTV, including center of mass (CoM) shifts and Hausdorff distance (HD) were extracted to quantify interfractional tumor variability over time. The 95th percentile of the HD was proposed as an estimate for framing integrated-tumor volume (ITV) margins. Results: The median residual tumor volume decreased to 63.57% (25th percentile: 54.08%; 75th percentile: 75.21%; 95th percentile: 107.7%) of the initial volume at T2 and to 49.24% (35.49%; 76.48%; 127.3%) at T3 of treatment. At T4, the median residual volume was 53.17% (32.90%; 72.93%; 94.33%). No regrowth was observed clinically. Median CoM shifts were 1.72 mm (95th percentile 5.45 mm) in the left–right direction, 2.73 mm (95th percentile 10.36 mm) in the anterior– posterior direction, and 5.52 mm (95th percentile 17.70 mm) in the cranio-caudal direction. The median HD was 13.65 mm, and the 95th percentile was 23.93 mm. Conclusion: Overall, tumor regression was seen, while slight GTV increases between two measurements were likely reflected minor redistribution within the cohort. Considerable GTV variabilty due to target motion and tumor regression were observed. Tumor motion predominantly occurred along the cranio-caudal axis. The relatively small CoM shifts compared to the HD
Conclusion: Although advanced registration methods significantly improve geometric accuracy and show lower accumulated gastrointestinal organ doses compared to simpler methods, this does not affect the reduction of total number of fractions with in-treatment dose adaptation compared to DVH summation. For this particular application, DVH summation remains practical and efficient, avoiding unnecessary complexity without compromising outcomes. References: Van Lieshout, E., Van Werkhoven, L. A., Hoogeman, M. S., Nout, R. A., Milder, M. T. W., & Nuyttens, J. J. M. E. (2025). Reducing number of treatment fractions for patients with abdominal lymph node oligometastases: the need for online adaptive radiotherapy to provide personalized adaptive fractionation. International Journal Of Radiation Oncology*Biology*Physics. https://doi.org/10.1016/j.ijrobp.2025.03.036 Keywords: OART, dose accumulation, fraction dose adaptation chemoradiotherapy for rectal cancer – First results from the prospective PRIMO study (NCT05524012) Mara Wiede 1,2 , Steffen Weimann 1 , Simon Lehmenkühler 1,3 , Daniel Güllmar 4 , Tobias Franiel 5 , Maximilian Römer 1,3 , Klaus Pietschmann 1,3 , Georg Wurschi 1,2 1 Department of Radiotherapy and Radiation Oncology, Jena University Hospital, Jena, Germany. 2 Interdisciplinary Center for Clinical Research, Jena University Hospital, Jena, Germany. 3 Comprehensive Cancer Center Central Germany, Partner site Jena, Jena, Germany. 4 Medical Physics Group, Institute for Diagnostic and Interventional Radiology, Jena University Hospital, Jena, Germany. 5 Institute of Diagnostic and Interventional Radiology, Friedrich- Digital Poster Highlight 2096 Interfractional tumor variability during
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