Impact of Interfractional Bladder and Trigone Displacement and Deformation on Radiation Exposure and Subsequent Acute Genitourinary Toxicity: A Post Hoc Analysis of Patients Treated with Magnetic Resonance Imaging–Guided Prostate Stereotactic Body Radiation Therapy in a Phase 3 Randomized Trial

Abstract

PurposeEmerging data suggest that trigone dosimetry may be more associated with post-stereotactic body radiotherapy (SBRT) urinary toxicity than whole bladder dosimetry. Herein, we quantify the dosimetric impact of interfractional displacement and deformation of the whole bladder and trigone during prostate SBRT using on-board, pre-treatment 0.35T MRI images. MethodsSeventy-seven 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. Bladder and trigone structures were contoured on images obtained from a 0.35T simulation MRI and five on-board pre-treatment MRIs. Dice Similarity Coefficient (DSC) scores and changes in volume between simulation and daily treatments were calculated. Dosimetric parameters including Dmax, D0.03cc, Dmean, V40Gy, V39Gy, V38Gy, and V20Gy for the bladder and trigone for the simulation and daily treatments were collected. Both physician-scored (CTCAE v4.03 scale) as well as patient-reported (IPSS and EPIC-26 scores) acute genitourinary (GU) toxicity outcomes were collected and analyzed. ResultsThe average treatment bladder volume was about 30% smaller than the simulation bladder volume; however, the trigone volume remained fairly consistent despite being positively correlated with total bladder volume. Overall, the trigone accounted for less than 2% of the bladder volume. Median DSC for the bladder was 0.79 while the median DSC of the trigone was only 0.33. No statistically significant associations between our selected bladder and trigonal dosimetric parameters and grade 2 or greater GU toxicity were identified, though numerically, patients with GU toxicity (grade ≥ 2) had higher intermediate doses to the bladder (V20Gy and Dmean) and larger volumes exposed to higher doses in the trigone (V40Gy, V39Gy, and V38Gy). ConclusionsThe trigone exhibits little volume change, but considerable interfractional displacement/deformation. As a result, the relative volume of the trigone receiving high doses during prostate SBRT varies substantially between fractions, which could influence GU toxicity and may not be predicted by radiation planning dosimetry.