Comparison of internal gross target volumes delineated on the maximum intensity projection of four-dimensional CT images and positron emission tomography-CT for primary thoracic esophageal cancer

Objective To compare volumetric size, conformity index （CI）, degree of inclusion （DI） of internal gross target volumes （IGTV） delineated on 4D-CT-MIP and PET-CT images for primary thoracic esophageal cancer. Methods Fifteen patients with thoracic esophageal cancer sequentially underwent enhanced 3D-CT, 4D-CT and PET-CT simulation scans. IGTVMIP was obtained by contouring on 4D-CT maximum intensity projection （ MIP）. The PET contours were determined with nine different threshold methods （SUV≥2.0, 2.5, 3.0, 3.5）, the percentages of the SUVmax（≥20%, 25%, 30%, 35%, 40%） and manual contours. The differences in size, conformity index （CI）, degree of inclusion （ DI） of different volumes were compared. Results The volume ratios （ VRs） of IGTVPET2. 5 to IGTVMIP , IGTVPET20% to IGTVMIP, IGTVPETMAN to IGTVMIP were 0.86, 0.88, 1.06, respectively, which approached closest to 1. The CIs of IGTVPET2.0,IGTVPET2.5,IGTVPET20%,IGTVPETMAN and IGTVMIP which were 0.55, 0.56, 0.56, 0.54,0.55, respectively, were significantly larger than other CIs of IGTVPET and IGTVMIP （Z= -3.408-2.215,P 〈0.05）. There were no statistical significance in the DIs of IGTVMIP and IGTVPET2.5,IGTVMIP and IGTVPET20%, IGTVMIP and IGTVPETMAN（0.77,0.82,0.71,0.67, 0.68,0.82,P〉0.05）. Conclusions The targets delineated based on SUV threshold setting of≥2.5, 20% of the SUVmax and manual contours on PET images correspond better with the target delineated on maximum intensity projection of 4D-CT images than other SUV thresholding methods. Key words: 18 F-FDG PET-CT; 4D-CT; Thoracic esophageal cancer; 18 F-FDG PET-CT; Four-dimensional computed tomography; Standardized uptake value; Internal gross target volume