An unusual case of thymic carcinoid causing Cushing's syndrome due to ectopic ACTH secretion detected by 18F-FDG PET/CT

Abstract

A 44-year-old female patient presented symptoms and laboratory data consistent with a Cushing’s syndrome. Endocrine investigations and negative pituitary imaging were suggestive of ectopic ACTH secretion. Therefore, the patient underwent fluorine18-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) and somatostatin receptor scintigraphy (SRS) searching for the cause of ectopic ACTH secretion. Before 18F-FDG injection, the patient had fasted for at least 6 h; at the time of the radiopharmaceutical injection she presented glucose blood levels corresponding to 90 mg/dL. Images were acquired one hour after intravenous injection of 270 MBq of 18F-FDG according to the body mass index. 18F-FDG PET/CT showed a focal area of increased radiopharmaceutical uptake in the anterior mediastinal region corresponding to a 2 cm nodule (Fig. 1). No other areas of abnormal increased 18FFDG uptake were detected in the rest of the body. Additionally, SRS did not show lesions with increased expression of somatostatin receptors. Based on the 18F-FDG PET/CT finding, the patient underwent biopsy of the mediastinal nodule. Histological examination showed the presence of atypical cells suggestive of a thymic neoplasia. At immunohistochemistry neoplastic cells were positive for chromogranin A and ACTH with low expression of somatostatin receptors, whereas the proliferative index (Ki67) was about 20%. A final histological diagnosis of atypical thymic carcinoid was performed. The patient underwent thymectomy and the postoperative course was favorable with clinical and biochemical remission of Cushing’s syndrome. Ectopic ACTH syndrome is a diagnostic challenge because it is often indistinguishable from Cushing’s disease. The role of 18F-FDG PET/CT in localization of tumors causing ectopic ACTH secretion is still controversial.1 Previous case reports demonstrated the usefulness of 18F-FDG PET only in detecting thymic carcinoids causing