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

Background: 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. Methods: 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. Results: 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). Conclusion: IGTV(MIP) and IGTV(2Phases) underestimate IGTVs. IGTV(MIP-Modified) is recommended to improve IGTV delineation in lung cancer.