S2848
RTT - RTT education, training, and advanced practice
ESTRO 2026
A total of 43 patients with pelvic tumors (23 rectal cancer, 17 cervical cancer, 3 endometrial cancer) who were treated in the Department of Radiation Oncology at Peking University Shenzhen Hospital between April 2020 and December 2021 were enrolled. All patients were positioned using a Bellyboard. One hour prior to CT simulation and each treatment session, patients consumed 500 ml of water for voluntary bladder filling. They were categorized into three groups based on bladder volume: 13 patients with a volume of <350 ml, 14 with 350-500 ml, and 16 with >500 ml. Before each treatment, a Bladderscan was used to ensure the bladder volume matched. Subsequently, cone-beam computed tomography (CBCT) was performed on the linear accelerator. The acquired CBCT images were registered with the digitally reconstructed radiographs (DRRs) from the treatment planning system to evaluate setup errors in three dimensions. Results: A total of 349, 353, and 459 image sets were acquired from the <350 ml, 350-500 ml, and >500 ml bladder volume groups, respectively. Comparative analysis revealed distinct setup error profiles among the groups. When comparing the <350 ml and 350-500 ml groups, significant differences (P < 0.05) were found in the vertical (VRT) and lateral (LAT) directions, but not in the longitudinal (LNG) direction or rotational (RTN) deviations. The <350 ml group demonstrated superior overall setup reproducibility. For the comparison between the >500 ml and 350-500 ml groups, significant differences (P < 0.05) were observed in VRT, LAT, and rotational deviations. The >500 ml group showed better reproducibility in the VRT and LAT directions but exhibited larger rotational errors. No significant differences (P > 0.05) in translational setup errors (VRT, LNG, LAT) were found between the <350 ml and >500 ml groups. However, the <350 ml group had significantly smaller rotational errors (P < 0.05).
Conclusion: Bladder volume significantly influences setup errors in patients receiving prone-position radiotherapy for pelvic tumors. The study indicates that patients with a bladder volume of either <350 ml or >500 ml after self- filling demonstrated smaller setup errors and superior setup reproducibility compared to those with a volume between 350-500 ml. Therefore, it is recommended in clinical practice to maintain a bladder volume below 350 ml for pelvic radiotherapy patients immobilized in the prone position to achieve optimal setup reproducibility. Keywords: Pelvic Radiotherapy ， Bladder volume ， Setup error Digital Poster Highlight 1096 Replacing the rescan? Dose evaluation of HyperSight CBCT for adaptive offline replanning in head & neck cancer Sjoerd Hoek, Paul van Duijvenvoorde, Yvonne van Herten, Abrahim Al-Mamgani, Anna Dinkla Department of Radiation Oncology, Amsterdam UMC, Amsterdam, Netherlands Purpose/Objective: Anatomical changes during radiotherapy for head and neck cancer can compromise dose delivery, traditionally necessitating repeat CT scans for offline re-planning. HyperSight CBCT provides fast imaging with Hounsfield Unit accuracy comparable to planning CT, enabling accurate offline re-planning while potentially eliminating repeat CTs. This may optimize workflow efficiency and reduce workload while maintaining dosimetric accuracy.
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