Use of 18F-FDG PET/CT to differentiate ectopic adrenocorticotropic hormone-secreting lung tumors from tumor-like pulmonary infections in patients with ectopic Cushing syndrome
Abstract Background: Ectopic adrenocorticotropic hormone (ACTH)-secreting lung tumors represent the most common cause of ectopic Cushing syndrome (ECS). Pulmonary opportunistic infections are associated with ECS and occasionally difficult to differentiate from tumors by using computed tomography (CT) alone. The present study aimed to evaluate the usefulness of 18F-fluorodeoxyglucose (FDG) positron emission tomography/CT (18F-FDG PET/CT) for differentiating ectopic ACTH-secreting lung tumors from tumor-like pulmonary infections in patients with ECS.Methods: We retrospectively reviewed the imaging data for 24 patients with ECS who were suspected to have ACTH-secreting lung tumors and underwent 18F-FDG PET/CT between 2008 and 2019. Part of the 24 patients underwent 99mTc-HYNIC-TOC scintigraphy and 68Ga-DOTA-TATE PET/CT.Results: In total, 18 patients had lung tumors and six had pulmonary infections. The primary source of ECS remained occult in the six patients with pulmonary infections. The maximum standardized uptake value (SUVmax) for pulmonary infections was significantly higher than that for tumors (P = 0.008). Receiver operating characteristic analysis was performed, and it was found that a cut-off SUVmax of 4.95 helped in differentiating lung tumors from infections with 75% sensitivity and 94.4% specificity. In a subgroup analysis of 12 typical and five atypical carcinoids, there was no significant between-group difference with respect to SUVmax, the lesion size, the ACTH level, and the prevalence of regional lymph node metastasis. Four out of 6 patients with 5 infectious lesions and 16 out of 18 patients with 16 ACTH-secreting tumors underwent 99mTc-HYNIC-TOC scintigraphy, and 1/6 patients with 1 infectious lesion, and 6 out of 18 patients with 6 ACTH-secreting tumors underwent 68Ga-DOTA-TATE PET/CT. There is no significant difference in sensitivity between tumor lesions and infections using 99mTc-HYNIC-TOC scintigraphy. Conclusions: Our findings suggest that pulmonary infections exhibit significantly higher FDG uptake than do well-differentiated ACTH-secreting lung tumors in 18F-FDG PET/CT. Therefore, SUVmax (cut-off 4.95) may be useful for differentiating the two conditions. However, 99mTc-HYNIC-TOC scintigraphy is of no value in distinguishing the focus of well-differentiated ACTH-secreting lung tumors from that of infection. Typical and atypical ACTH-secreting lung carcinoids may show similar clinical behavior and appearance on 18F-FDG PET/CT.