Evaluation of the outcome of lung nodules missed on 18F-FDG PET/MRI compared to 18F-FDG PET/CT in patients with known malignancies

The lower detection rate of ¹⁸F-fluordesoxyglucose positron emission tomography/magnetic resonance imaging (¹⁸F-FDG PET/MRI) compared to ¹⁸F-FDG PET/computed tomography (¹⁸F-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 ¹⁸F-FDG PET/MRI. Material and Methods: Fifty-one oncologic patients (mean age: 56.6±14.0 years; 29 female, 22 male; tumor stages: I (n = 7), II (n = 7), III (n = 9), IV (n = 28)) who underwent ¹⁸F-FDG PET/CT and subsequent ¹⁸F-FDG PET/MRI on the same day were retrospectively enrolled. Images were analyzed by two readers in random order and separate sessions with a minimum of four weeks apart. A maximum of ten lung nodules was identified for each patient on baseline imaging. Presence, size and presence of focal tracer uptake was noted for each lung nodule detected on ¹⁸F-FDG PET/CT and ¹⁸F-FDG PET/MRI using a postcontrast T1-weighted (T1w) 3 dimensional (3D) gradient echo (GRE) volume-interpolated breathhold-examination (VIBE) sequence with fat suppression (fs) as morphological dataset. Follow-up CT or ¹⁸F-FDG PET/CT (mean time-to-follow-up: 11 months, range: 3-35) was used as 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 ¹⁸F-FDG PET/CT, ¹⁸F-FDG PET/MRI detected 92 nodules. Accordingly, 42 lung nodules (average size: 3.9±1.3 mm; range: 2–7 mm) were missed by ¹⁸F-FDG PET/MRI. None of the missed lung nodules presented with focal tracer uptake on baseline imaging or follow-up ¹⁸F-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 four patients were rated malignant. As a result, one patient required upstaging from tumor stage I to IV. Conclusion: Although the majority of small lung nodules missed on ¹⁸F-FDG PET/MRI was 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.

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