Purpose: Urethral injury plays a significant role in genitourinary (GU) toxicities for prostate cancer patients receiving radiation therapy (RT). However, urethra sparing has been challenging due to non-trivial urethra delineation with conventional RT technologies. MRI provides strong soft tissue contrast and potentially high urethra visibility. Moreover, MR-guided online adaptive RT (MRgART) based on patient’s daily MRI shows the potential of improved organ-at-risk (OAR) sparing. In this study, we used MR-guided radiation therapy (MRgRT) on-board MRI to retrospectively analyze inter-fractional urethra localization and geometric variation for a cohort of prostate cancer patients and evaluated the value of MRgART for urethra sparing.
Methods: Eleven prostate cancer patients were scanned on a 0.35T MRgRT system using a urethra-specific T2-weighted 3D HASTE sequence at two different timepoints between simulation and the final fraction. The average time between urethral imaging pair was 19.8 days. A resident radiation oncologist retrospectively contoured the prostatic urethra for all patient images. Additionally, the oncologist qualitatively scored the prostatic urethra visibility. For each patient, the urethra imaging pairs were rigidly registered based on the prostate gland and urethra inter-fractional geometric variation was evaluated using 95th percentile Hausdorff distance (HD95), mean-distance-to-agreement (MDA), and DICE coefficient.
Results: Qualitatively, 3D HASTE provided good conspicuity and the urethra could be identified clearly for the majority of the patient scans. The average length and diameter of the prostatic urethra was 3.54±0.52cm and 0.78±0.12cm. The average HD95, MDA, and DICE between urethra imaging pairs was 0.26±0.10cm, 0.11±0.04cm, and 0.58±0.14. The majority of urethral inter-fractional shifts were observed in the anterior-posterior and superior-inferior direction.
Conclusion: Urethral inter-fractional geometric variation is significant, with respect to the urethra diameter, when shifts are introduced in the anterior-posterior direction. MRgART workflow with optimal on-board imaging may improve urethra sparing throughout treatment.
Funding Support, Disclosures, and Conflict of Interest: Dr. Yang has received honoraria and consulting fees from ViewRay. Dr. Kishan has received honoraria and research funding from ViewRay. Dr. Steinberg has received a consulting fee from ViewRay. Dr. Hu has received consulting fees and research funding from ViewRay.