3754 - Lateral Differences in Transposed Ovary Motion: A Large-Scale FBCT Analysis for Personalized PRV Margins in Cervical Cancer Radiotherapy

Purpose/Objective(s):

Ovarian transposition (OT) is critical for preserving ovarian function in young cervical cancer patients undergoing pelvic radiotherapy. However, positional uncertainty of transposed ovaries remains a key challenge. This study systematically analyzes three-dimensional(3D) motion patterns of bilateral transposed ovaries using high-resolution fan-beam CT(FBCT) and provides evidence-based support for personalized planning risk volume(PRV) design. This is the first large-scale study to characterize 3D motion and analyze lateral differences between left and right ovaries.

Materials/Methods: Twenty-five cervical cancer patients underwent bilateral ovarian transposition before pelvic radiotherapy (45-50Gy in 25-28 fractions)were enrolled. Pelvic radiotherapy was completed on a domestic CT-linear radiotherapy system. A total of 639 FBCT images were collected before treatment. Two senior radiation oncologists independently contoured bilateral transposed ovaries.Three-dimensional(3D) centroid displacements were calculated through deformable registration.Motion amplitudes, systematic errors, and random errors of both ovaries in superior-inferior(SI), anterior-posterior(AP), and left-right(LR) directions were analyzed. Statistical analysis was performed using paired t-tests for lateral comparisons and one-way ANOVA for directional differences.

Results:

Patient Characteristics:Mean age 32.5±5.3(23-41), FIGO stages IIA1-IIB.Motion Amplitude:Overall 3D motion amplitude was largest in SI(3.3±0.9mm), followed by AP(2.5±1.1 mm) and LR (2.0±1.0mm). Lateral comparison showed left ovary had 18% greater motion in SI than right (3.6±1.0mm vs 3.1±0.8mm) and 12% greater in AP (2.7±1.2mm vs 2.4±1.0mm), consistent with anatomical proximity to intestinal peristalsis.Systematic and Random Errors:Overall systematic errors were 1.8mm-SI, 1.4mm-AP, 1.1mm-LR. Random errors were 1.3 mm-SI, 1.2mm-AP, 1.0mm-LR.PRV Margin Recommendation: Based on van Herk formula, asymmetric PRV margins are recommended:5mm-SI, 4mm-AP, 3mm-LR for both ovaries. This achieves >95% coverage of ovarian while reducing normal tissue irradiation volume by 15-20% compared to traditional uniform margins.FBCT provided clear identification of transposed ovaries in all scans, significantly superior to conventional KV-CBCT/MVCT visualization.

Conclusion:

This first large-scale FBCT-based study establishes a comprehensive 3D motion database of bilateral transposed ovaries. Left transposed ovaries exhibit greater motion range. The recommended asymmetric PRV margins effectively protect bilateral ovarian function while minimizing normal tissue irradiation risk, providing practical guidance for individualized ovarian-sparing radiotherapy in young cervical cancer patients. Limitations include single-center design and moderate sample size. Future multi-center studies with long-term ovarian function follow-up are warranted to validate these findings.