Quantitative Assessment of an Intra-Fractional Online MR-Guided Adaptive Workflow: Stop, Scan, and Adapt
Abstract
Purpose
The Stop-Scan-Adapt (SSA) feature introduced with the MRIdian A3i upgrade enables intra-fractional adaptation during online MR-guided adaptive radiotherapy (ART) through re-optimization without patient repositioning. While SSA efficiently responds to intra-fractional anatomical changes, its dose accounting methodology and clinical interpretation are not well characterized. This study evaluates the SSA workflow, dose handling, and deformation-related dose uncertainty.
Methods
A retrospective MR-guided adaptive treatment plan was delivered to a motion phantom containing a moving insert as a target surrogate. Online adaptation followed target contour modification. Partial dose delivery preceded 1cm superior target displacement to simulate internal motion. SSA was initiated, a new MR image acquired, target-related structures updated, and re-optimization performed. Two delivery scenarios were evaluated: Scenario-1: 10% of dose delivered prior to SSA and Scenario-2: 30% delivered prior to SSA. Delivered MUs, SSA QA reports, and composite dose calculations were analyzed.
Results
In both scenarios, SSA excluded beams already delivered and re-optimized treatment using the remaining beams. Apparent discrepancies between monitor units reported in the SSA plan and SSA QA report were explained by the SSA dose-handling approach. SSA assigns half of the delivered pre-SSA dose to the pre-SSA image and the other half to the post-SSA image. The pre-SSA half is deformed onto the post-SSA image and summed with post-SSA dose, then with the SSA-optimized plan dose. Increasing pre-SSA delivered dose from 10% to 30% increased PTV D95% uncertainty from 1.6% to 6.5% between undeformed plan-sums and the SSA-reported composite dose, particularly in regions of induced target displacement.
Conclusion
SSA enables efficient intra-fractional adaptation but relies on deformation-based dose estimation that can introduce spatial dose uncertainty when substantial dose is delivered prior to SSA initiation. Understanding SSA dose accounting is essential for accurate plan evaluation and supports the need for structured team training when implementing SSA in MR-guided ART.