Developing and validating a simple urethra surrogate model to facilitate dosimetric analysis to predict genitourinary toxicity

Purpose The urethra is a critical structure in prostate radiotherapy planning; however, it is impossible to visualise on CT. We developed a surrogate urethra model (SUM) for CT-only planning workflow and tested its geometric and dosimetric performance against the MRI-delineated urethra (MDU). Methods The SUM was compared against 34 different MDUs (within the treatment PTV) in patients treated with 36.25 Gy (PTV)/40 Gy (CTV) in 5 fractions as part of the PACE-B trial. To assess the surrogate's geometric performance, the Dice similarity coefficient (DSC), Hausdorff distance (HD), mean distance to agreement (MDTA) and the percentage of MDU outside the surrogate (UOS) were calculated. To evaluate the dosimetric performance, a paired t-test was used to calculate the mean of differences between the MDU and SUM for the D99, D98, D50, D2 and D1. The D(n) is the dose (Gy) to n% of the urethra. Results The median results showed low agreement on DSC (0.32; IQR 0.21–0.41), but low distance to agreement, as would be expected for a small structure (HD 8.4 mm (IQR 7.1–10.1 mm)), MDTA 2.4 mm (IQR, 2.2 mm-3.2 mm). The UOS was 30 % (IQR, 18–54 %) indicating nearly a third of the urethra lay outside of the surrogate. However, when comparing urethral dose between the MDU and SUM, the mean of differences for D99, D98 and D95 were 0.12 Gy (p = 0.57), 0.09 Gy (p = 0.61), and 0.11 (p = 0.46), respectively. The mean of differences between the D50, D2 and D1 were 0.08 Gy (p = 0.04), 0.09 Gy (p = 0.02) and 0.1 Gy (p = 0.01), respectively, indicating good dosimetric agreement between MDU and SUMs. Conclusion While there were geometric differences between the SUM and MDU, there was no clinically significant difference between urethral dose-volume parameters. This surrogate model could be validated in a larger cohort and then used to estimate the urethral dose on CT planning scans in those without an MRI planning scan or urinary catheter.