S2128
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Purpose/Objective: Inter-fraction variations in bowel and bladder filling during radiotherapy for rectal cancer may lead to positional and dosimetric deviations of target volumes and organs at risk (OARs), compromising treatment accuracy. This study aimed to assess inter-fraction morphological consistency and dosimetric variations in target volumes and OARs using an integrated CT- Linac system, providing data to optimize the timing of adaptive radiotherapy interventions. Material/Methods: A retrospective analysis was conducted on 20 rectal cancer patients treated with 25-fraction IMRT to a total dose of 50 Gy.FBCT images were acquired at six time points: after the 1st, 5th, 10th, 15th, 20th, and 25th fractions. All images were non-rigidly registered to the planning CT, with target volumes (CTV and PTV) propagated across sequences andOARs(bowel and bladder) segmented using automated techniques. Contour consistency was evaluated using the Dice Similarity Coefficient (DSC) and Hausdorff Distance (HD), while volume changes were assessed via absolute volume measurements. Dosimetric parameters, including mean dose, dose-volume ratios, and maximum dose to specific volumes, were also analyzed. Paired comparisons were performed chronologically across nine groups, encompassing both adjacent (e.g., 5th vs. 10th) and cross-fraction (e.g., 1st vs. 25th) time points. Results: During the initial 1-10 fractions, both CTV and PTV demonstrated high stability, with DSC of 0.898 ± 0.028 and 0.928 ± 0.024, and HD of 8.94 ± 1.41 mm and 8.33 ± 2.38 mm. However, stability significantly declined beyond 15 fractions, with DSC decreasing to 0.813 ± 0.061 (CTV) and 0.859 ± 0.046 (PTV), while HD increased to 13.63 ± 2.84 mm and 13.39 ± 2.48 mm by the 20th fraction. Target volumes progressively decreased, though the D95 dose remained stable.OARs variability was notable, particularly for the bladder, which exhibited substantial morphological inconsistency (DSC: 0.52 ± 0.06, HD: 35.19 ± 1.98 mm) and a negative dose-volume correlation. At the 10th fraction, bladder volume was minimal (65.08 ± 21.35 cm ³ ) with peak mean dose (3171.15 ± 689.80 cGy), whereas by the 15th fraction, volume increased (135.18 ± 79.26 cm ³ ) with a concurrent dose reduction (3161.88 ± 564.32 cGy). The small intestine showed lower morphological consistency (DSC: 0.43 ± 0.02, HD: 48.96 ± 3.58 mm) and a significant negative dose-DSC correlation. In contrast, the femoral head remained stable throughout (DSC: 0.81 ± 0.03, HD: 10.72 ± 1.05 mm), with a consistent volume percentage receiving 2000 cGy (26.7 ± 3.5%).
Conclusion: Significant inter-fraction variations in target volumes and OARs (bladder, small intestine) were observed in rectal cancer radiotherapy, with target stability declining notably after 15 fractions. FBCT-guided adaptive radiotherapy may mitigate these variations. Keywords: Inter-fractional Variation,CT-Linac
Digital Poster 1588
Prospective evaluation and clinical outcomes of adaptive radiotherapy for locally advanced non- small cell lung cancer Ingvild Helø Carlsen 1 , Vilde Ragnvaldsen Dyvik 1 , Tuva Borgund Johannessen 1 , Kristine Fjellanger 1 , John-Vidar Hjørnevik 1 , Inger Marie Sandvik 1 , Turid Husevåg Sulen 1 , Tone Nybø 1 , Johanna Austrheim Hundvin 1 , Sara Margareta Cecilia Pilskog 1,2 , Øystein Fløtten 3,4 , Vilde Drageset Haakensen 5 , Liv Bolstad Hysing 1,2 1 Department of Oncology and Medical Physics, Haukeland University Hospital, Bergen, Norway. 2 Department of Physics and Technology, University of Bergen, Bergen, Norway. 3 Department of Thoracic Medicine, Haukeland University Hospital, Bergen, Norway. 4 Department of Clinical Science, University of Bergen, Bergen, Norway. 5 Department of Oncology, Oslo University Hospital, Oslo, Norway Purpose/Objective: Radiotherapy for locally advanced non-small cell lung cancer poses challenges due to anatomical changes during treatment. Adaptation (ART) improves precision and may enhance patient outcomes. In 2019 we implemented a traffic light-based adaptive protocol for
Made with FlippingBook - Share PDF online