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SU‐FF‐J‐173: Tumor Volume Delineation: A Comparison of Imaging Protocols for Lung Tumors

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<0.01), 0.88±0.08 (p<0.01), 0.64±0.13 (p<0.01), and 0.64±0.12 (p<0.01) times smaller than the ITV, respectively. The OI of the ITV with GTVFB, GTVMIP, GTVAIP, and GTVSCT was 60%, 81%, 61% and 64%, respectively. The average RMS distances of the GTVFB, GTVMIP, GTVAIP, and GTVSCT relative to the ITV were 0.44cm, 0.30cm, 0.45cm and 0.40cm, respectively. The OI of GTVFB with GTVMIP, GTVAIP, and GTVSCT was 62% 68% and 71%, respectively and the corresponding RMS distances were 0.34cm, 0.36cm and 0.35cm, respectively. Conclusions: ITV is statistically larger than GTVMIP, and they are both larger than GTVFB, GTVAIP and GTVSCT. Even though GTVFB, GTVAIP and GTVSCT have similar size, the surface mismatch among them is still distinguishable.