Solitary nodular invasive mucinous adenocarcinoma of the lung: Imaging diagnosis using the morphologic-metabolic dissociation sign

Objective: To evaluate the efficacy of the morphologic-metabolic (M-M) dissociation sign based on computed tomography (CT) and fluorine-18-fluorodeoxyglucose positron emission tomography (PET)/CT in discriminating invasive mucinous adenocarcinoma (IMA) from invasive non-mucinous adenocarcinomas (ADCs) of the lung. Materials and Methods: The Institutional Review Board approved this retrospective study. Among surgically resected solitary pulmonary nodule (SPN)-type ADCs (< 3 cm in diameter), 35 patients with IMAs and 329 with invasive non-mucinous ADCs were included. Morphologic malignancy was established if the tumor with lobulated or spiculated margin on CT presented a tumor shadow disappearance rate of < 0.5. The M-M dissociation sign was determined when a malignant-morphologic nodule on CT showed maximum standardized uptake value (SUVmax) < 3.5 on PET/CT. Results: Among 35 IMAs (size: 21 ± 7 mm, SUVmax: 1.8 ± 2.0) and 329 invasive non-mucinous ADCs (size: 21 ± 6 mm, SUVmax: 4.6 ± 4.2), the M-M dissociation sign was observed in 54% of IMAs (19/35) and 10% of invasive non-mucinous ADCs (34/329) (p < 0.001). The diagnostic performance of the sign in discriminating IMA from invasive non-mucinous ADCs showed a sensitivity of 54.3% (95% confidence interval [CI], 36.7-71.2), specificity 89.7% (95% CI, 85.9-92.7), positive predictive value 35.8% (95% CI, 26.5-46.5), and negative predictive value 94.9% (95% CI, 92.8-96.4). Multivariate analyses revealed metabolic benignity (odds ratio [OR] 2.99; 95% CI, 1.01-8.93; p = 0.047) and M-M dissociation sign (OR 6.35; 95% CI, 2.76-14.62; p < 0.001) to be significant predictors of SPN-type IMAs. Conclusion: Identification of the absence of M-M dissociation sign is an accurate indicator for excluding IMA from SPN-type lung ADCs.