A Quantitative Workflow to Reduce Vertical Setup Uncertainty In Dibh Radiotherapy Using the LAP Luna 3D Sgrt System
Abstract
Purpose
To evaluate a quantitative SGRT-based workflow for pre-imaging DIBH amplitude verification and quantify its impact on setup precision by comparing residual setup errors across the VRT, LNG, and LAT axes.
Methods
At CT simulation, a reference DIBH amplitude was recorded using the LAP LUNA 3D SGRT system's motion tracking ROI. At each treatment, patients were coached to match this amplitude (±5 mm tolerance) prior to any imaging. Following this SGRT verification, IGRT (CBCT and/or port films) was performed to determine residual setup errors. This workflow was implemented for four left-breast patients over 71 fractions. The residual translational couch shifts (VRT, LNG, LAT) from 36 fractions requiring non-zero corrections were analyzed via box plot to compare setup variability. The remaining 35 fractions required zero couch correction post-IGRT, representing days where the initial SGRT-guided setup required no correction.
Results
The mean deviation between the planned CT amplitude and the pre-IGRT amplitude was 0.62 mm, with coaching required in ~60% of fractions. The workflow also identified two patients unable to reproducibly meet the DIBH tolerance, who were replanned for a free-breathing technique, preventing potentially inaccurate DIBH delivery. A box plot analysis of the IGRT-derived couch shifts demonstrated that the VRT shifts exhibited the highest precision (median: -0.5 mm), in stark contrast to the significantly greater variability observed in the LNG and LAT couch shifts.
Conclusion
This SGRT-based workflow provides a robust, quantitative, and non-radiographic method to ensure the geometric reproducibility of DIBH prior to imaging. It functions as an effective pre-treatment screening tool, helping to identify and exclude patients who cannot consistently reproduce their breath-hold, thereby avoiding unnecessary imaging dose. The demonstrably lower variability in vertical couch shifts compared to the other axes provides definitive quantitative evidence that this SGRT-based amplitude verification workflow actively improves setup precision in the primary breath-hold dimension.