Evidence-Based Definition of Region of Interest (ROI) for Abdominal DIBH Surface-Guided Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> To define and evaluate the appropriate abdominal region of interest (ROI) as a surrogate of both body and diaphragm positioning in deep-inspiration breath-hold (DIBH) for surface-guided radiotherapy (SGRT) using 3D optical surface imaging (OSI). <h3>Materials/Methods</h3> Six simulated abdominal ROIs were evaluated to calculate the correlations with the diaphragm position using 4DCT images of 20 abdominal patients. Twelve points of interest (POIs) were defined (6 on the central soft tissue and 6 on the bilateral ribs) at 3 superior-inferior levels between the xiphoid and umbilicus, and different sub-groups represented different ROIs. ROI-1 was the largest, containing all 12 POIs while ROI-2 had only 8 inferior POIs, ROI-3 had 6 lateral POIs, and ROI-4 had 4 superior-lateral POIs over the ribs, ROI-5 contained 6 central and 2 most inferior-lateral POIs, and ROI-6 contained 6 central and 4 inferior-lateral POIs. The lateral body midlines defined the ROI coverages to represent body position. Internally, the right diaphragm dome positions were measured in 4DCT (0% and 50% phases). The Pearson correlation coefficients were calculated between the diaphragm and all 12 external POIs individually or grouped as 6 ROIs. The quality of the abdominal ROIs was evaluated as potential internal surrogates and, therefore, potential ROIs for SGRT DIBH setup. <h3>Results</h3> The four most inferior POIs show the highest mean correlation (r=0.75) with diaphragmatic motion, and the correlation decreases as POIs move superiorly. The mean correlations are higher at r=0.67 for ROI-2, r=0.64 for ROI-5, and r=0.63 for ROI-6, while ROI-1 has r=0.60, ROI-3 has r=0.50, and ROI-4 has only r=0.28. <h3>Conclusion</h3> This study demonstrates that the rectangular/triangular soft-tissue ROI is the optimal surrogate for body positioning and diaphragmatic motion, even when treating tumors under the rib cage. This evidence-based ROI definition should be utilized when treating abdominal cancers with FB and/or DIBH setup.