3055 - Multiple Rectal States Optimization as Novel Approach to Conquer Unique Uncertainty in Cervical Cancer Intensity-Modulated Proton Therapy
Presenter(s)
M. Li1, T. Dai2, Y. Yin2, and P. Xie3; 1Department of Radiation Oncology Physics and Technology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, jinan, China, 2Department of Radiation Oncology Physics and Technology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China, 3Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, Shandong, China
Purpose/Objective(s): This study aims to evaluate the impact of inter-fractional rectal physiological state changes on the plan robustness of cervical cancer intensity-modulated proton therapy (IMPT) and to establish a robust optimization strategy integrating multiple rectal states.
Materials/Methods: Patients with cervical cancer who underwent adjuvant RT or definitive RT between March 2024 and October 2025 were included. CT images (Group A: 20 patients with ‘filled’ and simulated ‘air’ states; Group B: 10 patients with ‘filled’, ‘natural empty’, and simulated ‘air’ states) were collected. First, nominal IMPT plans were created based on the rectal filled-state CT using three treatment planning strategies with different beam-angle arrangements. Second, robust optimization integrating filled/air states for Group A and filled/ natural empty/air states for Group B was performed for the same three strategies. Plan robustness was evaluated by calculating the dose parameter variation of the clinical target volume (CTV) and organs at risk (OARs) across different rectal states.
Results: Thirty cervical cancer patients were retrospectively included. All nominal plans showed CTV dose fluctuations under rectal state changes. The adoption of robust optimization improved target dose robustness for all strategies. In Group A, under Strategy 1, the median CTV D98% range decreased from 1.79 Gy to 0.39 Gy (reduction ~78%); the median CTV D95% range decreased from 0.80 Gy to 0.26 Gy (reduction ~67.5%). In Group B, for Strategy 1, the median CTV D98% range decreased from 12.22 Gy to 1.02 Gy (reduction ~91.6%); the median CTV D95% range decreased from 4.20 Gy to 0.40 Gy (reduction ~90.5%). After robust optimization, the preset target dose objective (D95% = 45 Gy) was achieved for most patients across all rectal states, without a significant increase in OAR doses.
Conclusion: Rectal state changes significantly affect the dosimetric robustness of IMPT plans for cervical cancer. We invented a novel optimization method, multiple rectal states optimization(MRSO), which effectively mitigates this uncertainty, ensuring reliable target coverage under various expected anatomical states without significantly increasing OAR doses. This approach provides a practical solution for managing inter-fractional rectal physiological changes in cervical cancer proton therapy.