Development of Prostate Bed Delineation Consensus Guidelines for Magnetic Resonance Image-Guided Radiotherapy and Assessment of Its Effect on Interobserver Variability

The use of MRI in radiotherapy planning is becoming more widespread, particularly with the emergence of MRI-guided radiotherapy (MRIgRT) systems. Existing guidelines for defining the prostate bed clinical target volume (CTV) show considerable heterogeneity. This study aims to establish baseline interobserver variability (IOV) for prostate bed CTV contouring on MRI, develop international consensus guidelines and evaluate its effect on IOV.Participants delineated the CTV on three MRI scans, obtained from the xxx xxx MR-Linac, as per their normal practice. Radiation oncologist contours were visually examined for discrepancies and interobserver comparisons were evaluated against Simultaneous Truth and Performance Level Estimate (STAPLE) contours using overlap metrics (Dice similarity coefficient and Cohen's Kappa), distance metrics (mean distance to agreement and Hausdorff distance) and volume measurements.. A literature review of post-radical prostatectomy local recurrence patterns was performed and presented alongside IOV results to the participants. Consensus guidelines were collectively constructed and IOV assessment was repeated using these guidelines.Sixteen radiation oncologists' contours were included in the final analysis. Visual evaluation demonstrated significant differences in the superior, inferior and anterior borders. Baseline IOV assessment indicated moderate agreement for the overlap metrics whilst volume and distance metrics demonstrated greater variability. Consensus for optimal prostate bed CTV boundaries was established during a virtual meeting. Post-guideline development, a decrease in IOV was observed. The maximum volume ratio decreased from 4.7 to 3.1 and volume coefficient of variation reduced from 40% to 34%. The mean Dice similarity coefficient rose from 0.72 to 0.75 and the mean distance to agreement decreased from 3.63mm to 2.95mm.Interobserver variability in prostate bed contouring exists amongst international genitourinary experts, although this is lower than previously reported. Consensus guidelines for MRI-based prostate bed contouring have been developed and this has resulted in an improvement in contouring concordance. However, IOV persists and strategies such as an education program, development of a contouring atlas and further refinement of the guidelines may lead to additional improvements.