Can We Avoid Ureters as Organs at Risk With MRI-Based Brachytherapy for Cervical Cancer? A Dosimetric Feasibility Study.

PURPOSE/OBJECTIVE(S) Ureteral stenosis (US) is an underreported complication of brachytherapy for cervical cancer, with limited data regarding dose constraints to reduce risk of toxicity. Previous work from our institution demonstrated that an EQD2 ≥ 77 Gy to 0.1 cm3 of ureter correlated with development of US. We sought to determine whether we can reduce ureter dose EQD2 ≤77 Gy while maintaining similar HR-CTV coverage. We also sought to determine if anatomical factors would limit achieving ureter dose constraints. MATERIALS/METHODS We identified patients with locally advanced cervical cancer treated with external beam radiation therapy plus HDR MRI-guided brachytherapy boost without hydronephrosis at diagnosis who had single or bilateral ureter dose EQD2 ≥ 77 Gy to 0.1 cm3, regardless of subsequent US. Replan was attempted for each individual brachytherapy fraction, aiming to achieve HR-CTV D90 ≥ 80-85 Gy without significant reduction in coverage, while maintaining rectal D2cc ≤65 Gy, bowel D2cc ≤65-70 Gy, sigmoid D2cc ≤70 Gy, and bladder D2cc ≤80 Gy. Distance of each ureter from lateral margin of HR-CTV and tandem were recorded, with mean values calculated on a per patient basis. Dependent samples T-test was performed to compare highest ureteral dose from original plan to replan ureteral dose and original plan HR-CTV D90 to replan HR-CTV D90. Independent samples T-test was performed to compare the mean distance of closest ureter to HR-CTV and tandem in patients for whom ureteral dose reduction was feasible to those for whom reduction was not feasible. RESULTS 25 patients met inclusion criteria, with all patients receiving 45 Gy in 25 fractions to the pelvis plus/minus paraaortic lymph nodes and 20 of 25 patients receiving mean parametrial dose of 5.4 Gy. Replan meeting ureteral dose of ≤77 Gy was feasible in 18 of 25 patients, with significant reduction in highest mean ureteral dose from 85.3 Gy to 76.7 Gy (P < 0.001). Mean original plan HR-CTV dose of 85.06 Gy decreased by 0.94 Gy to replan dose of 84.12 Gy in successfully replanned patients. Mean distance of closest ureter to HR-CTV margin was 1.5mm in patients for whom achieving ureter dose constraints was not feasible compared to 8.2mm for patients for whom replan was feasible (P = 0.038). Mean distance of closest ureter to tandem was 14.5mm for patients whom achieving ureter dose constraints was not feasible compared to 24.7mm for patients for whom replan was feasible (P < 0.001). Replan achieved ureter dose ≤77 Gy in the majority of patients who experienced US (56%), and 100% of unilateral US cases occurred in the ureter closest to HR-CTV. CONCLUSION When ureters are contoured, optimization to reduce ureter dose to ≤77 Gy is feasible, potentially allowing for reduction in US risk without significant compromise in HR-CTV D90. Anatomic variation of ureter position may impact this optimization. Ureters may be considered as potential organs at risk during MRI-based brachytherapy treatment if visualization is feasible to reduce risk of US. AUTHOR DISCLOSURE S.A. Koerner: Stock; Novavax, Pfizer, Inovio Pharmaceuticals, Ocugen, Teladoc, ADC Therapeutics, CVS, Viatris Inc. T. Baig: None. H. Kim: None. J.L. Rodriguez-Lopez: None. A. Keller: None. S. Beriwal: None.