Author: Minsong Cao, Amar Kishan, Yi Lao, An Liu, Beth Neilsen, X. Sharon Qi, Kun Qing, Ke Sheng, Michael Steinberg, Luca F Valle, Terence Williams 👨🔬
Affiliation: Department of Radiation Oncology, University of California, Los Angeles, Department of Radiation Oncology, City of Hope Medical Center, Department of Radiation Oncology, University of California, San Francisco, Department of Radiation Oncology, City of Hope National Medical Center 🌍
Purpose: To investigate the clinical relevance of planned versus delivered doses in toxicity associations utilizing localized assessment of genitourinary (GU) toxicity in bladder subregions among prostate cancer (PC) patients undergoing MR-guided radiotherapy (MRgRT).
Methods: 77 PC patients treated with 40 Gy in 5 fractions on a 0.35T MR Linac were enrolled. GU toxicities investigated included urinary frequency, retention, and dysuria. Bladder surface meshes were generated from contours delineated on planning and daily MRIs for all patients and fractions, with regional alignment through surface-based registration. Planned and delivered surface dose maps (SDMp and SDMd) were created by projecting doses onto bladder surface faces and accumulating fractional doses, respectively. On both SDMp and SDMd, face-wise Spearman’s rank tests with GU grades were performed to evaluate correlation with severity, followed by group t-tests to identify regions with significant differences between symptomatic (≥ grade 1) and asymptomatic patients. Finally, logistic regression was conducted to determine cut-off values in regions identified by both models.
Results: Both SDMp and SDMd models identified urinary frequency-related clusters on the anterior-superior bladder surface, with a mean cut-off value of 4.3 Gy. Spurious negative correlations were seen only in SDMp. Urinary retention clusters were found in the inferior mid-section, anterior to the urethra, with a mean cut-off value of 26.0 Gy, and SDMd identified additional trends in the posterior-superior regions. For dysuria, clusters were concentrated in the inferior-posterior trigone areas, with a mean cut-off value of 40.9 Gy, showing comparable patterns in both models.
Conclusion: A novel 3D bladder surface-based toxicity mapping framework was developed to evaluate bladder toxicity in PC MRgRT using both planned and delivered doses. Results were consistent between planned and delivered doses, with delivered dose models showing fewer spurious correlations. The established dose-toxicity mappings and the derived cut-off doses, may guide future treatment planning.