Click here to

Session: MR-Only Simulation and Targeting [Return to Session]

Urethral Inter-Fractional Geometric and Dosimetric Variations of Prostate Cancer Patients: A Study Using An On-Board MRI

J Pham*, R Savjani, S Yoon, T Yang, Y Gao, M Cao, P Hu, K Sheng, D Low, M Steinberg, A Kishan, Y Yang, University of California, Los Angeles, Los Angeles, CA


TU-F115-IePD-F2-1 (Tuesday, 7/12/2022) 1:15 PM - 1:45 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 2

Purpose: In this study, we used an MR-guided radiation therapy (MRgRT) system to retrospectively analyze urethral inter-fraction geometric and dosimetric variation for a cohort of prostate cancer patients and evaluated the value of on-board prostatic urethra visualization.

Methods: 26 prostate cancer patients were scanned on a 0.35T MRgRT system using a clinical bSSFP sequence and an optimized T2-weighted HASTE sequence at simulation and final fraction. Two residents (RO1 and RO2) contoured the urethras on all HASTE images. Simulation and final fraction HASTE were rigidly registered, and urethral inter-observer and inter-fractional geometric variation was evaluated using 95th percentile Hausdorff distance (HD95), mean-distance-to-agreement (MDA), center-of-mass shift (COMS), and DICE coefficient. For dosimetric analysis, simulation and final fraction HASTE were registered to their respective 3D bSSFP planning MRI. Both ROs’ urethra contours were transferred for treatment planning and prediction. MRgRT plans were generated to deliver 40 Gy to 95% of the PTV. Treatment plans were optimized to meet clinical constraints, including a urethral hotspot limiting constraint (V42Gy< 0.03cc). The dose was calculated and predicted on the bSSFPs with deformably registered electron density information from simulation CT using MRgRT treatment planning system. Urethral dose constraint change was evaluated.

Results: Average inter-observer HD95, MDA, COMS, and DICE was 2.85±1.34mm, 1.02±0.36mm, 3.16±1.61mm, and 0.58±0.15. Average inter-fraction HD95, MDA, COMS, and DICE was 3.26±1.54mm, 1.29±0.54mm, 3.34±2.01mm, and 0.49±0.18. All patient simulation MRgRT plans met clinical acceptance and urethral dose constraints. For RO1 and RO2, 23/26 (88%) and 21/26 (81%) patients’ final fraction predicted urethral dose did not meet the planned constraint. The average urethral V42Gy change was 0.48±0.58cc.

Conclusion: Urethral inter-fraction motion and anatomic change can result in daily treatment violating urethral hotspot limiting constraints. On-line MR-guided adaptive radiation therapy (MRgART) workflow with on-board urethral imaging may reduce urethral hotspots and, as a result, GU toxicity.

Funding Support, Disclosures, and Conflict of Interest: Dr.Yang received honoraria and consulting fees from ViewRay. Dr.Kishan received honoraria and research funding from ViewRay. Dr.Steinberg received a consulting fee from ViewRay. Dr.Hu received consulting fees and research funding from ViewRay. Dr.Low received consulting fees from ViewRay and a grant from Varian.


MRI, Prostate Therapy, Dosimetry


IM/TH- MRI in Radiation Therapy: MRI for treatment planning

Contact Email