Determination of patient-specific internal gross tumor volumes for lung cancer using four-dimensional computed tomography

To determine the optimal approach to delineating patient-specific internal gross target volumes (IGTV) from four-dimensional (4-D) computed tomography (CT) image data sets used in the planning of radiation treatment for lung cancers. We analyzed 4D-CT image data sets of 27 consecutive patients with non-small-cell lung cancer (stage I: 17, stage III: 10). The IGTV, defined to be the envelope of respiratory motion of the gross tumor volume in each 4D-CT data set was delineated manually using four techniques: (1) combining the gross tumor volume (GTV) contours from ten respiratory phases (IGTV_AllPhases); (2) combining the GTV contours from two extreme respiratory phases (0% and 50%) (IGTV_2Phases); (3) defining the GTV contour using the maximum intensity projection (MIP) (IGTV_MIP); and (4) defining the GTV contour using the MIP with modification based on visual verification of contours in individual respiratory phase (IGTV_MIP-Modified). Using the IGTV_AllPhases as the optimum IGTV, we compared volumes, matching indices, and extent of target missing using the IGTVs based on the other three approaches. The IGTV_MIP and IGTV_2Phases were significantly smaller than the IGTV_AllPhases (p < 0.006 for stage I and p < 0.002 for stage III). However, the values of the IGTV_MIP-Modified were close to those determined from IGTV_AllPhases (p = 0.08). IGTV_MIP-Modified also matched the best with IGTV_AllPhases. IGTV_MIP and IGTV_2Phases underestimate IGTVs. IGTV_MIP-Modified is recommended to improve IGTV delineation in lung cancer