Correlation of PET standard uptake value and CT window-level thresholds for target delineation in CT-based radiation treatment planning

Purpose: To develop standardized correlates of [18 F]fluoro-2-deoxy- d -glucose positron emission tomography (FDG-PET) standard uptake value (SUV) to computed tomography (CT)-based window and levels. Methods and Materials: Nineteen 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 ≥ 2.5, and regions of 40% maximum SUV were analyzed. Results: The mean SUV was 9.3. Mean GTV volumes were 253 cc for CT, 221 cc for SUV ≥ 2.5, and 97 cc for SUV40%Max. Average volume difference was −259% between ≥2.5 SUV and CT and −162% between SUV40%Max and CT. Percent difference between GTV ≥ 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. Conclusions: Our 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 7, differences between the SUV threshold filters remain constant. Because of variability in volumes obtained by using SUV40%Max, we recommend using SUV ≥ 2.5 for radiotherapy planning in non–small-cell lung cancer.