• Robert Hong, James Halama, Davide Bova, Anil Sethi, and Bahman Emami.
• Department of Radiation Oncology, Nuclear Medicine Division, Loyola University Medical Center, Maywood, IL 60153, USA.
• Int. J. Radiat. Oncol. Biol. Phys. 2007 Mar 1; 67 (3): 720-6.

PurposeTo develop standardized correlates of [18F]fluoro-2-deoxy-d-glucose positron emission tomography (FDG-PET) standard uptake value (SUV) to computed tomography (CT)-based window and levels.Methods And MaterialsNineteen patients with non-small-cell lung cancer who underwent imaging with positron emission tomography (PET) and CT were selected. A method of standardizing SUV within CT planning software was developed. A scale factor, determined by a sensitivity calibration of the PET scanner, converts voxel counts to activity per gram in tissue, allowing SUVs to be correlated to CT window and levels. A method of limiting interobserver variations was devised to enhance "edges" of regions of interest based on SUV thresholds. The difference in gross tumor volumes (GTVs) based on CT, PET SUV >or= 2.5, and regions of 40% maximum SUV were analyzed.ResultsThe mean SUV was 9.3. Mean GTV volumes were 253 cc for CT, 221 cc for SUV >or= 2.5, and 97 cc for SUV40%Max. Average volume difference was -259% between >or=2.5 SUV and CT and -162% between SUV40%Max and CT. Percent difference between GTV >or= 2.5 SUV and SUV40%Max remained constant beyond SUV > 7. For SUVs 4-6, best correlation among SUV thresholds occurred at volumes near 90 cc. Mean percent change from GTVs contoured according to CT (GTV CT) was -260% for GTV2.5 and -162% for GTV40%Max. Using the SUV40%Max threshold resulted in a significant alteration of volume in 98% of patients, while the SUV2.5 threshold resulted in an alteration of volume in 58% of patients.ConclusionsOur method of correlating SUV to W/L thresholds permits accurate displaying of SUV in coregistered PET/CT studies. The optimal SUV thresholds to contour GTV depend on maximum tumor SUV and volume. Best correlation occurs with SUVs >6 and small volumes <100 cc. At SUVs >7, differences between the SUV threshold filters remain constant. Because of variability in volumes obtained by using SUV40%Max, we recommend using SUV >or= 2.5 for radiotherapy planning in non-small-cell lung cancer.

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