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Purpose: To investigate differences in tumor contours delineated on free breathing (FB), four dimensional (4D), maximum intensity projection (MIP), average intensity projection (AIP), and slow‐CT (SCT) images.Methods and Materials: Data from ten patients who underwent stereotactic body radiation therapy of lungcancers were retrospectively investigated. For each patient, a FB CT and a 4D CT scan was acquired during simulation. Following the scan, MIP and AIP images were reconstructed. Since the 4D CT scan was acquired at a low pitch, a slow‐CT scan was also reconstructed. These scans were repeated prior to each treatment, resulting in 48 CTdata sets. The GTV was delineated on FB, MIP, AIP, and SCT images and was compared with the internal target volume (ITV), which comprised the union of GTVs delineated on each phase of the 4DCT. Three evaluation metrics were used for contour comparison: GTV volume, overlap index (OI), and root‐mean‐squared (RMS) distance. Results: On average GTVFB, GTVMIP, GTVAIP, and GTVSCT volumes were 0.66±0.13 (p |
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