Purpose To investigate the correlations in target volumes based on 18F-FDG PET/CT and four-dimensional CT (4DCT) to detect the feasibility of implementing PET in determining gross target volumes (GTV) for tumor motion for primary thoracic esophageal cancer (EC). Methods Thirty-three patients with EC sequentially underwent contrast-enhanced 3DCT, 4DCT, and 18F-FDG PET-CT thoracic simulation. The internal gross target volume (IGTV)10 was obtained by combining the GTV from ten phases of 4DCT. The GTVs based on PET/CT images were defined by setting of different standardized uptake value thresholds and visual contouring. The difference in volume ratio, conformity index (CI), and degree of inclusion (DI) between IGTV10 and GTVPET was compared. Results The images from 20 patients were suitable for further analysis. The optimal volume ratio of 0.95±0.32, 1.06±0.50, 1.07±0.49 was at standardized uptake value (SUV)2.5, SUV20%, or manual contouring. The mean CIs were from 0.33 to 0.54. The best CIs were at SUV2.0 (0.51±0.11), SUV2.5 (0.53±0.13), SUV20% (0.53±0.12), and manual contouring (0.54±0.14). The mean DIs of GTVPET in IGTV10 were from 0.60 to 0.90, and the mean DIs of IGTV10 in GTVPET ranged from 0.35 to 0.78. A negative correlation was found between the mean CI and different SUV (P=0.000). Conclusion None of the PET-based contours had both close spatial and volumetric approximation to the 4DCT IGTV10. Further evaluation and optimization of PET as a tool for target identification are required.
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