TH‐C‐17A‐10: A Novel Volumetric Respiratory Surrogate Using Optical Surface Imaging

Purpose: Respiration induces motion of the entire torso because the semifluid internal organs are movable driven primarily by the diaphragm within the connected body cavities: thoracic, abdominal and pelvis. Therefore, surface motion of the entire torso represents all respiration‐induced external motion and could serve as a more informative respiratory surrogate. Methods: Four‐dimensional optical surface (4DOS) imaging was used to monitor the entire torso as a volumetric respiratory surrogate. High speed image capture yields high spatial‐resolution images at 5 frames per second using 3 ceiling‐mounted stereoscopic cameras, capturing all moving surface of the torso. 4DOS images were retrospectively reconstructed. An in‐house MATLAB program was designed to automatically process the surface images with 1×1mm 2 grid and calculate torso volume variation as a function of time with a common volume of interest defined by the torso surface, a posterior cut‐plane (PCP), and 2 vertical cut‐planes at superior‐inferior borders. Torso volume variation during quiet breathing (<±3mmHg) represents time‐resolved tidal volume (TV) since the tissues and reserved body air within the torso conserve. The spatial distribution of TV in the torso is quantified as breathing pattern (BP=ΔVthorax/TV) using the rib cage as the thorax. Two volunteers were examined performing 4 different breathing patterns, including fake breathing during breath hold. Results: Using different PCPs at or below sagittal midline of the torso, 4DOS produces same TV (<±2%) in free‐/chest‐/belly‐breathing, suggesting that posterior body motion is negligible. Fake breathing during breath hold shows large BP variations (e.g., −ΔVthorax=ΔVabdomen≤750cm 3 ) but small TV changes (−6±39cm 3 ), depicting volume conservation. Surface motion is location‐dependent, suggesting intrinsic uncertainties and limited usefulness of point‐fiducial surrogates. Conclusion: The 4DOS‐based respiratory surrogate produces dynamic TV and BP values, which are potentially useful in developing a more reliable tumor motion surrogate. Further study of this tidal‐volume surrogate with more volunteers is on‐going under an IRB‐approved protocol. This research is in part supported by NIH (U54CA137788/132378).