Comparison of primary target volumes delineated on four-dimensional CT and (18) F-FDG PET/CT of non-small-cell lung cancer

BACKGROUND: To determine the optimal threshold of (18) F-fluorodexyglucose ((18) F-FDG) positron emission tomography CT (PET/CT) images that generates the best volumetric match to internal gross target volume (IGTV) based on four-dimensional CT (4DCT) images. METHODS: Twenty patients with non-small cell lung cancer (NSCLC) underwent enhanced three-dimensional CT (3DCT) scan followed by enhanced 4DCT scan of the thorax under normal free breathing with the administration of intravenous contrast agents. A total of 100 ml of ioversol was injected intravenously, 2 ml/s for 3DCT and 1 ml/s for 4DCT. Then (18) F-FDG PET/CT scan was performed based on the same positioning parameters (the same immobilization devices and identical position verified by laser localizer as well as skin marks). Gross target volumes (GTVs) of the primary tumor were contoured on the ten phases images of 4DCT to generate IGTV(10). GTV(PET) were determined with eight different threshold using an auto-contouring function. The differences in the position, volume, concordance index (CI) and degree of inclusion (DI) of the targets between GTV(PET) and IGTV(10) were compared. RESULTS: The images from seventeen patients were suitable for further analysis. Significant differences between the centric coordinate positions of GTV(PET) (excluding GTV(PET15%)) and IGTV(10) were observed only in z axes (P < 0.05). GTV(PET15%), GTV(PET25%) and GTV(PET2.0) were not statistically different from IGTV(10) (P < 0.05). GTV(PET15%) approximated closely to IGTV(10) with median percentage volume changes of 4.86%. The best CI was between IGTV(10) and GTV(PET15%) (0.57). The best DI of IGTV(10) in GTV(PET) was IGTV(10) in GTV(PET15%) (0.80). CONCLUSION: None of the PET-based contours had both close spatial and volumetric approximation to the 4DCT IGTV(10). At present 3D-PET/CT should not be used for IGTV generation.