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

Area of Science:

• Radiotherapy
• Medical Imaging
• Computational Anatomy

Background:

• Accurate urethra delineation is crucial for prostate radiotherapy planning.
• The urethra is not visible on CT scans, complicating treatment planning.
• A CT-only planning workflow requires a method to estimate urethral position and dose.

Purpose of the Study:

• To develop and evaluate a surrogate urethra model (SUM) for CT-only prostate radiotherapy planning.
• To assess the geometric and dosimetric performance of the SUM against the MRI-delineated urethra (MDU).

Main Methods:

• The SUM was compared to 34 MDUs in patients undergoing prostate radiotherapy (PACE-B trial).
• Geometric accuracy was assessed using Dice similarity coefficient (DSC), Hausdorff distance (HD), and mean distance to agreement (MDTA).
• Dosimetric performance was evaluated by comparing dose-volume parameters (D99, D98, D50, D2, D1) between MDU and SUM using paired t-tests.

Main Results:

• Median DSC was 0.32, with low HD (8.4mm) and MDTA (2.4mm), indicating expected geometric discrepancies for a small structure.
• 30% of the MDU was outside the SUM (UOS), highlighting geometric limitations.
• Despite geometric differences, mean differences in D99, D98, and D95 were not clinically significant (p>0.46).
• Good dosimetric agreement was observed for D50 (0.08Gy, p=0.04), D2 (0.09Gy, p=0.02), and D1 (0.1Gy, p=0.01).

Conclusions:

• The SUM demonstrates no clinically significant difference in urethral dose-volume parameters despite geometric variations.
• The SUM can be a valuable tool for estimating urethral dose in CT-only planning when MRI is unavailable.
• Further validation in a larger cohort is recommended before widespread clinical adoption.