TU-F-BRF-07: Accuracy of Routine Treatment Planning 4D and DIBH CT Delineation of the Left Anterior Descending Artery in Radiotherapy

Purpose: To assess the feasibility of routine treatment planning 4DCT and deep inspiration breath‐hold (DIBH) to accurately contour the left anterior descending artery (LAD), a primary indicator of cardiac toxicity, for radiotherapy treatment planning of breast cancer. Methods: Ten subjects were imaged with a cardiac‐gated MRI protocol to determine the displacement of a ROI that included the LAD. The subjects performed a series of breath‐hold maneuvers to obtain short‐axis and radial views, which were resampled to create a 3D‐volume. Tissue motion was determined using a multi‐resolution 3D optical flow deformable image registration algorithm. The ROI motion was then used as a spatial boundary to characterize the blurring motion of the LAD in ten patients during clinical 4DCT and DIBH protocols. A radiologist contoured the LAD. Coronary motion‐induced blurring artifacts were quantified by applying an unsharp filter to accentuate the LAD despite motion‐blurring. The 4DCT maximum inhalation and exhalation respiratory phases were co‐registered to determine the LAD displacement during tidal respiration, as visualized in 4DCT. Results: The average 90 th percentile heart motion for the ROI was 0.7±0.1mm(LR), 1.3±0.6mm(SI), 0.6±0.2mm(AP) in the cardiac‐gated MRI cohort. The average relative increase in the number of voxels comprising the LAD contour was 69.4±4.5% for the DIBH. During tidal respiration, the average relative increase in the LAD contour was 69.3±5.9% and 67.9±4.6% for inhalation and exhalation respiratory phases respectively. The average 90 th percentile LAD motion was 4.8±1.1mm(LR), 0.9±0.4mm(SI), 1.9±0.6mm(AP) for the 4DCT cohort, in the absence of cardiac‐gating. Conclusion: Uncompensated coronary motion was the dominant form of motion blurring present in the CT images due to the high frequency of the cardiac cycle relative to the respiratory cycle. The 4D and DIBH CT contour delineation of the LAD was consistently overestimated without cardiac‐gating, which could have led to inaccurate dose volume histogram indicators in clinical practice.