S2115
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
preparation and patient instructions may improve adaptive stability and potentially reduce the frequency of replanning, which should be further investigated. Keywords: PotD, MIBC
marked, and a simultaneous integrated boost (SIB) technique is applied using the full-bladder CT for treatment planning. Despite this adaptive approach, replanning is frequently required. This study aimed to identify the reasons for adaptation and to assess how variations in bladder and rectal filling contribute to the need for replanning. Material/Methods: This retrospective study included 17 consecutive MIBC patients (mean age 74 years; 13 men, 4 women) treated with the PotD approach between January 2024 and April 2025. Clinical data from CBCT images, replanning CT scans, treatment log files, and bladder volume measurements in RayStation were reviewed. Bladder and rectal filling, anatomical displacements, and plan selection patterns reflecting the day-to-day choice of library plans based on the patient’s anatomy were analyzed. Differences between replanned and non-replanned patients were assessed using the Mann–Whitney U test, with descriptive parameters summarizing bladder volume and rectal cross- sectional data. Results: Replanning was required in 5 of 17 patients (29%). Bladder volumes on CBCT were generally smaller than on the pCT (p = 0.032). Cranial and ventral bladder shifts were most frequently observed, causing anatomical variations during treatment and discrepancies between the planned and delivered dose distributions. These patients showed a smaller average bladder volume decrease ( − 65 cm ³ vs − 166 cm ³ ) (Figure 1). While stable bladder filling is desirable, cranial and ventral shifts often necessitated larger CTVs to maintain adequate coverage, resulting in higher doses to adjacent OARs. In addition, large rectal shifts (deviation score 0.67 cm vs 0.45 cm) were associated with replanning, indicating a displaced rectal position on CBCT compared with the pCT (Figure 2).
Mini-Oral 1023 A dose accumulation consistency error metric for image-guided radiotherapy Lando Bosma 1 , Mario Ries 2 , Cornel Zachiu 1 , Koen Kuijer 1 , Guus Grimbergen 1 , Bas Raaymakers 1 1 Radiotherapy department, UMC Utrecht, Utrecht, Netherlands. 2 Imaging division, UMC Utrecht, Utrecht, Netherlands Purpose/Objective: Online-adaptive image-guided radiotherapy enables daily visualisation of a patient's anatomy, allowing organ delineation and treatment planning for each treatment fraction. To estimate the total delivered dose, dose accumulation can be performed by aligning daily images using (deformable) image registration. For clinical implementation, evaluating the accuracy of the accumulated dose is crucial. We therefore introduce the dose accumulation consistency error (DACE) as a novel metric for this purpose. Material/Methods: Our metric is based on performing dose accumulation multiple times, using each fraction’s anatomy as reference. The consistency of these accumulated doses is then quantified using dose-volume histograms (DVHs) based on the independent delineations of each fraction. Our DACE is defined as the mean absolute deviation of the DVH-parameters from these accumulated doses. A diagrammatic visualisation for 3 fractions is shown below.
Conclusion: The PotD approach managed daily anatomical variations, but replanning was still required in nearly one-third of patients. Cranial and ventral bladder shifts and rectal deviations caused geometric variations, necessitating larger CTVs and resulting in higher OAR doses. Applying anisotropic margins for full-bladder treatments and optimizing bladder
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