2020 - Comparing Urethral Delineation for MRI-Based and Urethrogram-Based Contouring
Presenter(s)
N. Bhargava1, Q. Wang1, D. R. Schmidt1, J. A. Aronovitz1, K. Sahani1, S. Suy2, S. P. Collins2, I. D. Kaplan1, and N. Aghdam1; 1Department of Radiation Oncology, Beth Israel Deaconess Medical Center, Boston, MA, 2Department of Radiation Oncology, University of South Florida (USF) Health Morsani College of Medicine, Tampa, FL
Purpose/Objective(s): Urethral dose is considered a key factor associated with toxicity in prostate cancer stereotactic body radiation therapy (SBRT). Although contrast-enhanced urethrogram delineation is widely regarded as the most consistent and reliable method, logistical constraints and patient comfort have prompted the use of MRI-based delineation as a more scalable alternative. This study aimed to evaluate the volumetric and dosimetric differences between urethrogram-based contours and retrospective MRI-based contours in patients undergoing SBRT.
Materials/Methods: From November 2024 to January 2025, 30 patients treated with SBRT at a high-volume academic center were retrospectively analyzed. During CT simulation, a urethrogram was performed via penile injection of contrast-infused lidocaine gel. A planning MRI was acquired for all patients and subsequently fused with the CT images. For treatment planning, the urethra was contoured based on the urethrogram, while an MRI-based contour was generated retrospectively. Dosimetric parameters—including urethral volume, V40Gy, D0.03cc, and D0.3cc—were extracted and compared between the two methods. Two-sample t-tests were used to compare differences in the methods. Additionally, MRI-based contour volumes from a large institutional database were compared with those obtained using the urethrogram-based method.
Results: MRI-based contouring yielded significantly higher dosimetric and volumetric values compared to the original urethrogram-based method. The mean V40Gy was 3.93cc versus 1.36cc (p < 0.05), D0.03cc was 43.9Gy versus 41.5Gy (p < 0.05), D0.3cc was 43.0 Gy versus 40.9 Gy (p < 0.05), and the total urethral volume was 5.19cc versus 2.48cc (p < 0.05) for MR-based and Urethrogram-based contouring, respectively. Additionally, significant difference exists in Urethrogram-based (n=443 patients) vs. MR-based (n=300 patients) urethra volumes, at 1.68cc vs. 4.64cc, respectively (p<0.05).
Conclusion: Although MRI-based urethral delineation offers advantages in terms of scalability and patient comfort, it produces significantly different volumetric and dosimetric outcomes compared to the more reliable contrast-enhanced urethrogram method. These differences may impact urethral dose constraints and toxicity profiles in prostate cancer SBRT specifically when considering intraprostatic boost, highlighting the need for standardized delineation protocols to optimize treatment outcomes.