Clinical use of 18F-FDG PET/CT in the differential diagnosis of patients with primary and secondary adenoid cystic carcinoma of the lung: a retrospective cohort study

Background: Adenoid cystic carcinoma (ACC) of the lung (ACCL) is a rare malignancy and includes primary ACCL (PACCL) and secondary ACCL (SACCL) metastasized from the ACC of the head and neck. Fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) has been shown to be useful in the differential diagnosis between primary and metastatic lung lesions. This study retrospectively investigated the role of 18F-FDG PET/CT in combination with clinicopathological findings in the management of patients with primary or secondary ACCL. Methods: Clinicopathological characteristics and 18F-FDG PET/CT metabolic parameters of 29 patients with PACCL and 11 patients with SACCL with pathological confirmation as gold standard were retrospectively collected. The association between PET/CT metabolic parameters and clinicopathological features was explored. Receiver operating characteristic (ROC) curve analysis was used to determine the optimal maximum standard uptake value (SUVmax) cut-off value to distinguish PACCL from SACCL. The prognostic value of PET/CT metabolic parameters and clinicopathological features was evaluated by Cox regression analysis. Results: SACCL patients more often presented with multiple ACC lesions in the peripheral lung (81.8% vs. 17.2%, P<0.05) and tended to be asymptomatic compared to patients with PACCL (72.7% versus 27.6%, P<0.05). The SUVmax was significantly higher in PACCL patients compared to SACCL patients (median 4.4 vs. 2.8, P<0.05). Furthermore, at a cut-off value of 3.2 for SUVmax, the sensitivity and specificity of 18F-FDG PET/CT in distinguishing PACCL from SACCL were 82.8% and 72.7%, respectively. Higher SUVmax of ACCL was observed in patients with clinical symptoms, fewer ACC lesions, or larger tumor size (P<0.05). The median PFS of ACCL patients was 71.1 months, and the 12- and 24-month PFS rates were 96.1% and 91.6%, respectively. Univariate Cox regression analysis showed that SACCL from metastasis, lesions located at the peripheral lung or multiple ACC lesions present in the lung, were associated with poorer PFS (P<0.05). However, multivariate Cox regression analysis showed that none of the variables examined were independent predictors of prognosis. Conclusions: The use of 18F-FDG PET/CT in combination with assessment of clinicopathological features was helpful in distinguishing PACCL from SACCL, which could provide guidance for clinical decision making.