Improvement in sensitivity with delayed imaging of pulmonary lesions with FDG-PET

This study was undertaken to determine the value of using dual-time point 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) imaging to distinguish malignant from benign pulmonary lesions after lesion detection by conventional computed tomography chest imaging.

Patients referred for characterization of lung lesions were included in this prospective study. Eighty-three patients had histopathologic confirmation of disease. Patients underwent FDG-PET coincidence imaging, performed with a dual-headed gamma camera at 1 h ("early" scan) and 3 h ("late" scan) after injection of 185 MBq of FDG. Studies were read independently by 2 physicians who had knowledge of the lesion location but not the final diagnosis. For both early and late images, readers graded FDG lesion uptake intensity on a scale of 1 (definitively benign) to 5 (definitively malignant) and classified studies dichotomously for malignancy. Tumor-to-background (T:B) ratios were computed using contralateral lung sites as controls.

Sixty one lesions (74 %) were non-small cell lung cancer, and 10 (12 %) were other primary tumors or metastases. Twelve lesions (14 %) were benign. T:B ratios were significantly higher for early versus late scans (+ 5.1 +/- 4.9 versus + 8.2 +/- 8.7, p = 0.01, n = 71) for malignancies but not for benign lesions (+ 3.1 +/- 3.4 versus + 2.6 +/- 2.2, n = 12). The percent change of T:B ratios was higher for malignant than benign lesions (+ 48.3 +/- 40.2 % versus + 7.2 +/- 22.8 %, p = 0.0009). No malignant lesion of any type demonstrated a time-decrease in FDG T:B ratios. The accuracy and sensitivity of lesion characterization were significantly higher for late scans than early scans for dichotomous visual readings. Quantitative analysis was found to provide significantly higher sensitivity and accuracy than visual analysis for lesion characterization, with no significant difference in test specificity.

In malignant pulmonary nodules, there is a progressive, although variable, increase in FDG uptake over time. Increasing FDG uptake is a nonspecific finding, as some benign lesions also demonstrate increasing uptake, particularly those associated with granulomas. The use of late PET images increases the accuracy and sensitivity of visual detection of malignancy.