High False Positivity in Positron Emission Tomography is a Potential Diagnostic Pitfall in Patients with Suspected Adrenal Metastasis

Although 18F-fluorodeoxyglucose (FDG) positron emission tomography combined with computed tomography (PET/CT) is a potentially powerful, non-invasive imaging tool in differentiating adrenal metastasis from benign disease, some adenomas also exhibit high FDG uptake, therefore mimicking metastasis (i.e., false positives). We aimed to evaluate the accuracy of FDG-PET/CT based exclusively on histology and to identify risk factors for adrenal metastasis. Among the 289 consecutive patients who underwent adrenalectomy, 39 (78.0 %) patients had suspected solitary adrenal metastasis and had a positive preoperative FDG-PET/CT. The FDG-PET/CT findings were correlated with the histology of the excised adrenal gland. To identify risk factors for adrenal metastasis, characteristics were compared between patients with histologically proven adrenal metastasis and those without. Youden’s index was used to calculate the optimal cut-off value for predicting adrenal metastasis. Histology of the excised adrenal tumor confirmed adrenal metastasis in 28/39 (71.8 %) patients while non-metastatic lesions comprised mostly benign adrenal cortical adenoma (n = 10) and one non-functional pheochromocytoma. Therefore, the overall false-positive rate of FDG-PET/CT was 28.2 %. History of primary lung malignancy [odds ratio (OR) (95 % CI) 20.00 (1.01–333.3), p = 0.049] and SUVmax > 2.65 [OR (95 % CI) 31.606 (2.46–405.71), p = 0.008] were independent risk factors for adrenal metastasis. Single adrenal uptake on FDG-PET/CT in suspected solitary adrenal metastasis was associated with a high false-positive rate (28.2 %). Risk factors associated with adrenal metastasis included a history of known primary lung malignancy and a SUVmax > 2.65 at the adrenal lesion of interest on FDG-PET/CT. Based on these findings, a new algorithm was constructed.