S2045
Physics - Image acquisition and processing
ESTRO 2026
margins (formula: 2.5 Σ + 0.7 σ ) were analyzed and compared with cervical target margins (7-10mm). Results: Transposed ovaries exhibited 3D displacement, with the largest amplitude in the SI direction (3.5±1.0mm), followed by AP (2.8±1.2mm) and LR (2.1±0.9mm). Random errors ( σ ) were: SI 1.8mm, AP 1.3mm, LR 1.0mm; systematic errors ( Σ ) were: SI 2.1mm, AP 1.7mm, LR 1.2mm. Maximum displacement ranges were: SI 8.2mm, AP 6.5mm, LR 4.9mm, significantly smaller than cervical target displacement.Margin Recommendation: The calculated PRV margins were SI=5mm, AP=4mm, and LR=3mm. This scheme ensures the actual ovarian position is within the PRV in 90% of fractions while avoiding excessive expansion that increases normal tissue exposure.Clinical Comparison: Ovarian displacement was 40%-65% lower than cervical targets, attributed to: absence of uterine deformation and reduced bladder filling effects; stable fixation via peritoneal suturing; and 0.5mm real-time positioning accuracy of FBCT. Conclusion: This study establishes the first large-sample motion database of transposed ovaries using high-frequency FBCT scans, confirming significantly smaller displacement amplitude than cervical targets. The recommended 3D asymmetric PRV margins (SI 5mm, AP 4mm, LR 3mm) balance ovarian protection and treatment optimization, reducing high-risk organ irradiation volume by 15%-20% compared to traditional uniform margins. These results provide kinematic support for personalized dose constraints (e.g., Dmax ≤ 5Gy) in OS-CRT, particularly for fertility preservation in patients ≤ 35 years. The high-precision imaging capability of United Imaging FBCT is critical to this breakthrough, and future studies should explore adaptive radiotherapy combined with age-stratified dose constraints. Keywords: Fan-beam CT,Transposed ovaries,Margin Capability of material differentiation in photon- counting CT: a comparison with dual-energy CT Tin Lok Chiu, Chi Wai Cheung, Cheuk Lam Kelly Chung, Fu Ki Yeung, Hoi Man Lau, Ho Ming Poon, Siu Ki Yu Medical Physics Department, Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong Purpose/Objective: Accurate material differentiation is crucial for particle therapy dose calculation, where CT images are used to convert CT numbers to stopping-power ratios (SPR). Photon-counting CT (PCCT) enables estimation of key physical quantities, such as relative electron density ( ρ e) and effective atomic number (Zeff ). This study Digital Poster Highlight 584
free oART. References: [1] Thomas SJ. Relative electron density calibration of CT scanners for radiotherapy treatment planning. Br J Radiol. 1999 Aug;72(860):781-6.[2] Van de Kleut ML, et al. Evaluating the use of diagnostic CT with flattening filter free beams for palliative radiotherapy: Dosimetric impact of scanner calibration variability. J Appl Clin Med Phys-2025.[3] Nelissen KJ, et al. Same-day adaptive palliative radiotherapy without prior CT simulation: Early outcomes in the FAST-METS study. Rt Oncol-2023. [4] Schuurhuizen CSEW, et al. Clinical feasibility of treatment planning on a diagnostic CT scan without or with single fraction plan adaptation in patients with stage II/III rectal cancer. Rt Oncol-2025. Keywords: HU-to-density, diagnostic CT, simulation- free oART Digital Poster 553 Study on the Motion Characteristics of Transposed Ovaries Using the Fan-beam CT of the United Imaging uRT-linac 506c Linear Accelerator Xiaolong Cheng 1 , Bourama SANGARE 2 , Lu Zhang 3 , Jiping Liu 1 1 Radiation Physics, Zhejiang Cancer Hospital, Hangzhou, China. 2 Radiation Oncology, Hospital in Mali, Bamako, Mali. 3 Radiation Oncology, First People's Hospital of Aksu Prefecture, Aksu, China Purpose/Objective: Ovarian-sparing cervical cancer radiotherapy (OS-CRT) is critical for improving quality of life in young cervical cancer patients. Although ovarian transposition (OT) reduces radiation exposure, the positional uncertainty
of transposed ovaries during fractionated radiotherapy remains a challenge. Current
international research lacks reliable motion data due to low CT/MRI scanning frequency ( ≤ 5 scans/patient) and small sample sizes (n<10). This study is the first to systematically quantify the 3D motion characteristics of transposed ovaries using high-resolution fan-beam CT (FBCT) integrated with United Imaging uRT-linac 506c, aiming to provide evidence-based support for planning risk volume (PRV) margin expansion in OS- CRT. Material/Methods: Eighteen patients with cervical cancer who underwent radical surgery and bilateral OT received pelvic radiotherapy (45-50Gy/25-28 fractions). FBCT scans (468 total) were performed before each treatment. Ovarian contours were manually delineated, and centroid displacements in LR, AP, SI directions were calculated via deformable registration. Random error ( σ ), systematic error ( Σ ), displacement range, and PRV
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