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 [F-18]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 < 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 >= 2.5 for radiotherapy planning in non-small-cell lung cancer. (c) 2007 Elsevier Inc.