The diagnostic role of dual phase F-18 FDG PET/CT in characterization of solitary pulmonary nodules

Purpose: Our objective was to evaluate the diagnostic role of dual phase F-18 FDG PET/CT in the characterization of the solitary pulmonary nodules (SPN). We also compared the SUV variables adjusted to body surface area (BSA), lean body mass (LBM) and blood glucose level (GLC) in the terms of decision making in patients with SPN. Inter and intraobserver variabilities in between nuclear medicine physicians during the interpretation of SPN and the effect of low dose non-enhanced CT information about SPN were also analysed.Methods: A total of 36 SPN in 36 patients (15 female; 21 male; age range, 32-83 years) were included in this retrospective study. The final diagnosis was confirmed by histopathologically or follow-up by CT. Two PET/CT scans were performed one (early) and two hours (delayed) after injection of F-18 FDG. The CT was evaluated by an experienced radiologist on thorax radiology and PET/CT images were interpreted by two experienced nuclear medicine physicians. Indeterminate SPN group was reevaluated with low dose nonenhanced CT information by nuclear medicine physician thus the additional value of low dose non enhanced CT was obtained. SUV values (early and delayed SUVmax and SUVmean adjusted to body weight; SUVmax and SUVmean adjusted to BSA, LBM and GLC); retention index (RI); nodule to mediastinum (nodule activity/subcarinal roi activity) ratios (nod/med) were calculated. The sensitivity, specificity, negative (NPV) and positive predictive values (PPV) and accuracy of early and delayed F-18 FDG PET/CT (both visual and semiquantitative assessments) and low dose non-enhanced CT were calculated. Intra and interobserver variabilities in between nuclear medicine physicians were analysed for early and delayed PET/CT images. ROC curves were obtained for each SUV parameter, nod/med ratio and RI. The threshold values were calculated in the terms of highest sensitivity and specificity points.Results: Histopathological evaluations and follow up of the patients revealed 16 patients had malignant tumour whereas 20 patients had benign lesions. There was no statistical difference between malign and benign SPN patients in the terms of height, injection dose of F18 FDG, weight, blood glucose, BSA and LBM values. The median (min-max) SUVmax values was 1.5 (0.5-4.1) in benign group and 4 (1.3-38) in malign SPN group. Early and delayed SUVmax, SUVmean, BSA-SUVmax, BSA-SUVmean, LBM-SUVmax, LBM-SUVmean, GLC-SUVmax, GLC-SUVmean ve nod/med values were statistically higher in malign SPN group than benign group. The sensitivity, specificity, NPV, PPV and accuracy of low dose non-enhanced CT was 88%, 50% and 67% respectively. The information of the low dose non-enhanced CT features of the SPN did not affect in decision of nuclear medicine physicians. With the treshold value of early SUVmax as 2.5 (standart published SUVmax threshold value in the literaturre) and 3.05 which is obtained from our population using ROC curve, 94-75% sensitivity, 75-80% specificity and 83-78% accuracy were calculated respectively. With the same threshold values for delayed scan, 100-88% sensitivity, 75-85% specificity and 86-86% accuracy were obtained respectively. The delayed BSA-SUVmax and LBM-SUVmax had 100% sensitivity, 80% specificity in the determining SPN characterization. Interobserver variability between nuclear medicine physicians significantly decreased in the delayed phase of PET/CT scan.Conclusion: The study showed that dual Phase PET/CT may increase the diagnostic potential of F-18 FDG PET in the characterization of SPN. Dual phase PET/CT also significantly decreased interobserver variability. BSA, LBM and glucose corrected SUV values did not have better diagnostic performance compared to routine body weight adjusted SUVmax values. SUVmax as 2.5 for early images and SUVmax as 3.05 for delayed images were acceptable threshold values in the characterization of SPN by F-18 FDG PET/CT. In this particular study group, a threshold value cannot be determined for retention index, but higher retention index values may show higher malignant potential in solitary pulmonary nodules.