Evaluation of the Outcome of Lung Nodules Missed on 18F-FDG PET/MRI Compared with 18F-FDG PET/CT in Patients with Known Malignancies

The lower detection rate of F-18-FDG PET/MRI than F-18-FDG PET/CT regarding small lung nodules should be considered in the staging of malignant tumors. The purpose of this study was to evaluate the outcome of these small lung nodules missed by F-18-FDG PET/MRI. Methods: Fifty-one oncologic patients (mean age SD, 56.6 +/- 14.0 y; 29 women, 22 men; tumor stages, I [n = 7], II [n = 7], Ill [n = 9], IV [n = 28]) who underwent F-18-FDG PET/CT and subsequent F-18-FDG PET/MRI on the same day were retrospectively enrolled. Images were analyzed by 2 interpreters in random order and separate sessions with a minimum of 4 wk apart. A maximum of 10 lung nodules was identified for each patient on baseline imaging. The presence, size, and presence of focal tracer uptake was noted for each lung nodule detected on F-18-FDG PET/CT and F-18-FDG PET/MRI using a postcontrast T1-weighted 3-dimensional gradient echo volume-interpolated breath-hold examination sequence with fat suppression as morphologic dataset. Follow-up CT or F-18-FDG PET/CT (mean time to follow-up, 11 mo; range, 3-35 mo) was used as a reference standard to define each missed nodule as benign or malignant based on changes in size and potential new tracer uptake. Nodule-to-nodule comparison between baseline and follow-up was performed using descriptive statistics. Results: Out of 134 lung nodules found on F-18-FDG PET/CT, F-18-FDG PET/MRI detected 92 nodules. Accordingly, 42 lung nodules (average size SD, 3.9 +/- 1.3 mm; range, 2-7 mm) were missed by F-18-FDG PET/MRI. None of the missed lung nodules presented with focal tracer uptake on baseline imaging or follow-up F-18-FDG PET/CT. Thirty-three out of 42 missed lung nodules (78.6%) in 26 patients were rated benign, whereas 9 nodules (21.4%) in 4 patients were rated malignant. As a result, 1 patient required upstaging from tumor stage I to IV. Conclusion: Although most small lung nodules missed on F-18-FDG PET/MRI were found to be benign, there was a relevant number of undetected metastases. However, in patients with advanced tumor stages the clinical impact remains controversial as upstaging is usually more relevant in lower stages.