Correlation of PET standard uptake value and CT window-level thresholds for target delineation in CT-based radiation treatment planning.
OBJECTIVE
To 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
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 >or= 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 >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.


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 >or= 2.5 for radiotherapy planning in non-small-cell lung cancer.
Hong
Robert
R
Department of Radiation Oncology, Nuclear Medicine Division, Loyola University Medical Center, Maywood, IL 60153, USA.
Halama
James
J
Bova
Davide
D
Sethi
Anil
A
Emami
Bahman
B
D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D019788:Fluorodeoxyglucose F18 / Q000493:pharmacokinetics; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D015588:Observer Variation; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000379:methods / Q000592:standards; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods / Q000592:standards
D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18