Dosimetric robustness of urethra sparing in ultrahypofractionated prostate SBRT using catheter-based motion tracking

PURPOSE: In ultrahypofractionated prostate stereotactic body radiation therapy (SBRT), genitourinary toxicity is the primary concern. Urethra sparing is a strategy to mitigate potential negative effects of higher single doses to the urethra and bladder neck. We report on the dosimetric robustness of urethra sparing using a catheter-based real-time monitoring device to track both interfractional variability and intrafractional motion. METHODS: In 12 patients undergoing prostate ultrahypofractionated radiotherapy (UHFX), intrafractional prostate motion was monitored using a HypoCath catheter (Micropos Medical™, Gothenburg, Sweden), enabling urethra contouring on the planning CT and cone-beam (CB)CT images used for image-guided radiation therapy (IGRT). Dose coverage of both the urethra and the prostate clinical target volume (CTV) was calculated and compared between the planning CT and matched CBCTs. In addition, prostate motion was simulated and the cumulative dose coverage of prostate and urethra was calculated as a function of time. RESULTS: The deviation between the planned and delivered urethral dose strongly correlated with elapsed time. Variations in the urethral curvature between planning CT and CBCT had only a minimal effect on the dose distribution. The combined effects of deviations in the urethral shape and prostate movement resulted in a mean dose deviation of 76 cGy for the urethral D1 of all fractions and patients. The patient who experienced the greatest prostate motion showed an increase in urethral D1 of 1.94 Gy (+5.2%). CONCLUSION: Urethra sparing is feasible and only marginally impaired by variations in urethral shape but is sensitive to longer treatment durations due to intrafractional movement. Direct visualization of the urethra is not essential for IGRT, but intrafractional motion control is recommended.