S2168
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
with concurrent cisplatin, followed by tandem and ovoid brachytherapy. Daily pre-treatment CBCT (n=150) scans were registered to planning CT images, and maximal uterine displacements were recorded in anterior–posterior (AP), superior–inferior (SI), and right–left (RL) directions. Bladder volumes were measured, and Pearson correlations with motion were calculated. Results: The study included six patients with a mean age of 54.50 ± 15.00 years. A total of 150 CBCT scans (25 scans per patient) were acquired and analyzed to assess uterine movement. Mean uterine displacement was greatest in the AP direction (0.96 ± 0.22 cm), followed by SI (0.92 ± 0.17 cm) and RL (0.90 ± 0.13 cm). The largest recorded shifts were 1.27 cm (AP), 1.30 cm (SI), and 1.19 cm (RL). Mean bladder volume during treatment was 330.75 ± 52.95 cm ³ . AP motion showed a moderate, non-significant correlation with bladder volume (r = 0.357, p ≈ 0.08). A trend was observed between increased rectal diameter and greater uterine displacement, particularly in the anterior-posterior (AP) direction. Patients with mean rectal diameters >3.5 cm demonstrated higher AP motion (mean 1.22 ± 0.38 cm) compared to those with rectal diameters ≤ 3.5 cm (mean 0.94 ± 0.29 cm). Uterine motion also demonstrated trends with age, BMI, and parity, with younger, higher-BMI, and multiparous patients exhibiting greater displacement. Motion variability peaked during mid-treatment fractions (10–20), coinciding with fluctuations in bladder and rectal volumes. Toward the final fractions, motion patterns appeared to stabilize, possibly due to improved patient adaptation to preparation routines and reduced tumor burden.
deformably registered to CBCTs (wCTs) to account for anatomical variations and positioning discrepancies. Treatment plans were re-optimised and recalculated on wCTs and sCTs for dosimetric comparison. Results: Median relative differences in DVH parameters for target volumes between sCTs and wCTs were minimal (0.16–0.35% - Table 1), with the largest for D98% (0.35%) and the smallest for Dmean (0.16%). Median gamma pass rates were 99.75% (2%/2 mm) and 99.94% (3%/3 mm) at a 1% low-dose threshold (Table 2), confirming excellent dosimetric agreement. Conclusion: This study demonstrates the feasibility and dosimetric accuracy of AI-generated sCTs from CBCT images in head and neck cancer radiotherapy. The proposed method enables high-fidelity, quantitative image generation, supporting its integration into adaptive workflows for organ-at-risk delineation, dose simulation, and treatment replanning to enhance personalised and efficient radiotherapy. Keywords: Synthetic-CT, adaptive, head and neck Quantification and Predictors of Uterine Motion Assessed by Daily Cone Beam CT Scan During Radiotherapy For Cervical Cancer Maham Khan, Laraib Khan, Sehrish Abrar, Mariam Hina, Tooba Ali, Fabiha Shakeel, Nasir Ali Radiation Oncology, Aga Khan University Hospital, Karachi, Pakistan Purpose/Objective: Cervical cancer treatment with external beam radiotherapy (EBRT) is challenged by uterine motion, which can compromise target volume coverage. Daily cone-beam computed tomography (CBCT) offers a Digital Poster 3855 means to quantify and adapt for inter-fraction displacement. This study aims to quantify inter- fraction uterine motion in patients with intact cervical cancer using daily CBCT and identify clinical and anatomical predictors influencing this displacement. As a pilot effort from a tertiary care centre in a low- and middle-income country (LMIC), it shares practical experience with motion-adaptive radiotherapy in a resource-constrained setting. Material/Methods: This prospective observational study enrolled six cervical cancer patients undergoing definitive EBRT
Conclusion: Uterine motion during EBRT for cervical cancer is direction-dependent and influenced by bladder and
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