FDG-PET/CT-based gross tumor volume contouring for radiation therapy planning: An experimental phantom study

As there is continuing controversy over the role of f-18-fluorodeoxyglucose (fdg)-positron emission tomography (pet)/ct-fused imaging in radiation therapy (rt) planning,we performed a phantom study to assess the feasibility of fdg-pet/ct-based gross tumor volume (gtv) contouring. the phantom set,consisting of an elliptical bowl and 6 spheres measuring from 10-37 mm in diameter,were filled with fdg to obtain 3 source-to-background ratios (sbrs) of 4,8,and 16. the ratio to maximum intensity at 5% intervals was applied as the threshold for contouring. the ratio between contoured- and actual volumes (volume ratio) was calculated,and the threshold ratio was selected to provide a volume ratio close to 100%. to consider the clinical application,we applied the threshold value (maximum intensity × threshold ratio) for the largest 37-mm sphere to the 6 spheres. the threshold ratio and the volume ratio in 6 spheres with 3 sbrs were compared using the friedman test. threshold ratios ranged from 25-80%; they were higher for smaller spheres (p = 0.003) and lower sbrs (p < 0.001). the volume ratios with the threshold value for the largest 37-mm sphere were lower in smaller spheres (p = 0.010). these results suggest that smaller lesions and higher background activities require a higher threshold ratio and smaller lesions a lower threshold value. fdg-pet/ct-fused imaging should not be used as a single modality but rather to obtain supplemental information in rt planning. the contoured gtv should be adjusted based on clinical data including conventional images.