Dosimetric Impact of Interfractional Bladder and Trigone Deformations: A Post-Hoc Analysis of a Phase III Randomized Trial of MRI-Guided Vs. CT-Guided Stereotactic Body Radiotherapy

Purpose/Objective(s) Emerging data suggest that dosimetry of the trigone may be more associated with post-SBRT urinary toxicity than whole bladder dosimetry. Herein, we quantify the dosimetric impact of interfractional deformations during SBRT for the whole bladder and trigone using on-board 0.35T MRI images. Materials/Methods Twenty consecutive patients treated with MRI-guided prostate SBRT (40 Gy/5 fractions) on the MRI arm of a phase III single-center randomized trial were included. All patients were encouraged to have a full bladder for treatment. Bladder and trigone structures were contoured on images obtained from a 0.35T simulation MRI and five on-board pre-treatment MRIs. Dice Similarity Coefficients (DSCs) and changes in volume, Dmax, and Dmean for the bladder and trigone were calculated. Results Overall, the trigone accounted for only 2% of the bladder volume. Median DSC for bladder was 0.83 (0.60-0.97) while the median DSC of trigone was only 0.39 (0-0.84) (p<0.001). No statistically significant changes in bladder or trigone volume were seen, though the trigone volume was significantly associated with bladder volume (p<0.0001). Quantitatively, the Dmax values for the bladder and trigone were relatively consistent, with only 2% relative change. In contrast, the Dmean values for the bladder and the trigone exhibited more variability (p<0.0001). Conclusion The trigone is an extremely small urinary substructure (∼2% of the bladder volume) that exhibits significant interfractional deformation. Minimal changes in Dmax for both structures are consistent with expectations given their high proximity to the target volume and the use of bladder preparation. However, constraints for the trigone are unexplored, and the very low DSC suggests that the relative volume of this substructure receiving higher doses (∼75-100% of the prescription dose) could vary substantially. A rigorous exploration of this concept for the full cohort of trial patients who completed MRI-guided SBRT is planned.