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Dual-Phase ¹⁸F-FDG PET in the Diagnosis of Pulmonary Nodules with an Initial Standard Uptake Value Less Than 2.5

¹ Department of Nuclear Medicine, National Cheng Kung University Medical Center, 138 Sheng-Li Rd., Tainan 704, Taiwan.
² Department of Radiology, National Cheng Kung University Medical Center, Tainan, Taiwan.
³ Department of Psychiatry, National Cheng Kung University Medical Center, Tainan, Taiwan.

OBJECTIVE. A cutoff standard uptake value (SUV) of 2.5 has been commonly adopted for ¹⁸F-FDG PET to evaluate pulmonary lesions, but false results can occur. Studies have shown the usefulness of delayed PET for improving accuracy. This study was designed to examine the efficiency of delayed PET of pulmonary nodules with an initial mean SUV less than 2.5.

MATERIALS AND METHODS. Dual-phase FDG PET studies were conducted with imaging 1 and 2 hours after FDG injection, and pulmonary lesions with an initial mean SUV less than 2.5 were identified. Nodules with pathologic reports were included in the study. The differences in mean SUV, maximal SUV, and retention index between benign and malignant pulmonary lesions were analyzed. Receiver operating characteristic analysis was performed to evaluate the discriminating validity of the retention index.

RESULTS. A total of 31 lesions (15 benign, 16 malignant) were included in the study. Among the benign lesions, 12 were granulomatous inflammation, including 10 tuberculosis lesions and two cryptococcosis lesions, and three were focal fibrosis. A retention index greater than 0% was observed in 87% of the benign lesions; 60% of the benign lesions had a retention index greater than 10%. Among the malignant lesions, 75% had a retention index greater than 0%, and 62% had a retention index greater than 10%. We found no significant differences in mean SUV, maximal SUV, and retention index between benign and malignant lesions. The area under the receiver operating characteristic curve did not differ from 0.5.

CONCLUSION. Delayed FDG PET is not useful for differentiating benign and malignant pulmonary nodules with an initial mean SUV less than 2.5 in geographic regions with epidemic granulomatous disease such as tuberculosis or in patients at high risk of granulomatous inflammation.

Keywords: delayed imaging • FDG • inflammation • lung nodules • malignancy

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