Feasibility of Therapist‐Driven MR‐Guided Adaptive Radiotherapy for Oligometastatic Disease: Geometric Accuracy and Dosimetric Impact

Introduction MRI‐guided adaptive radiation therapy (ART) is the resource‐intensive process of daily treatment plan modification. This study aims to demonstrate the feasibility of radiation therapist (RT)‐led MRI‐guided ART for oligometastatic disease (OMD) by comparing geometric accuracy and dosimetric differences between RT and radiation oncologist (RO) re‐contouring. Methods Five RTs and five ROs retrospectively re‐contoured gross target volumes (GTVs) and organs‐at‐risk (OARs) for eight OMD cases. RT and RO contours were compared against consensus RO Simultaneous Truth and Performance Level Estimation (RO‐STAPLE) contours using the Dice similarity coefficient (DICE), mean distance to agreement (MDA), planning target volume (PTV) D95 and OAR D0.5cc using the Wilcoxon signed‐rank test. Moreover, an RO qualitatively scored all contours using a 5‐point Likert scale. Results We found very good geometric accuracy with average (±standard deviation) GTV DICE of 0.82 ± 0.06 for RTs and 0.85 ± 0.09 for ROs and MDA of 0.88 ± 0.03 mm for RT and 0.75 ± 0.05 mm for ROs relative to the RO‐STAPLE. Qualitative GTV Likert scores were excellent, 4.8/5 for RTs and 4.7/5 for ROs. Mean percent difference in PTV D95 compared to RO‐STAPLE was small but significantly higher for RTs (0.5% ± 1.5%) compared with ROs (−0.7% ± 1.9%, p < 0.05). Mean relative change in OAR D0.5cc results was small with −1% ± 6% for RTs and −1% ± 12% for ROs. Conclusions Here we provide the first report of geometric and dosimetric contouring uncertainty for MR‐guided online ART for OMD. Our results show that RT re‐contouring maintains similar performance for eligible targets and OARs compared with RO contours, establishing the initial feasibility of an RT‐led workflow.