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The Role of [¹⁸F]Fluorodeoxyglucose Positron Emission Tomography and [¹¹¹In]-Diethylenetriaminepentaacetate-D-Phe-Pentetreotide Scintigraphy in the Localization of Ectopic Adrenocorticotropin-Secreting Tumors Causing Cushing’s Syndrome

Pediatric and Reproductive Endocrinology Branch, National Institute of Child Health and Human Development (K.P., I.I., L.K.N.), and Nuclear Medicine Department, Clinical Center (C.C.C., J.A.C., M.W.), National Institutes of Health, Bethesda, Maryland 20892-1583

Address all correspondence and requests for reprints to: Dr. Karel Pacak, Unit on Clinical Neuroendocrinology, Pediatric and Reproductive Endocrinology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Building 10, Room 9D42, 10 Center Drive, Bethesda, Maryland 20892-1583. E-mail: karel{at}mail.nih.gov.

Conventional imaging modalities cannot localize the source of ACTH in 30–50% of patients with Cushing’s syndrome (CS) caused by ectopic ACTH secretion (EAS). We prospectively evaluated whether [¹⁸F]fluorodeoxyglucose (FDG) positron emission tomography (PET) or [¹¹¹In]-diethylenetriaminepentaacetate-D-Phe-pentetreotide (OCT) at higher than standard doses of radionuclide (18 mCi; H-OCT), can detect these tumors. Seventeen patients with presumed EAS based on inferior petrosal sinus sampling results underwent routine anatomical imaging studies [computed tomography (CT) and magnetic resonance imaging (MRI)] and OCT scintigraphy with 6 mCi (L-OCT). Research studies included FDG-PET in all patients and H-OCT if L-OCT was negative. ACTH-secreting tumors were localized in 13 patients and were occult in four. Nine of 17 CT, six of 16 MRI, six of 17 FDG-PET, eight of 17 L-OCT, and one of nine H-OCT studies were true positives. The sensitivity of CT and combined H- and L-OCT scintigraphy was higher (both 53%; 95% confidence interval, 29–76%) than that of MRI (37%; 95% confidence interval, 16–64%) or FDG-PET (35%; 95% confidence interval, 15–61%). FDG-PET did not detect tumors that were occult on CT/MRI. L-OCT was a useful complementary modality to CT and MRI. As H-OCT identified a tumor in one patient with otherwise negative imaging, it should be considered only when other imaging modalities fail to localize the ACTH-secreting tumor in patients with EAS.

Abbreviations: CS, Cushing’s syndrome; CT, computed tomography; DTPA, diethylenetriaminepentaacetate; EAS, ectopic ACTH secretion; FDG, [¹⁸F]fluorodeoxyglucose; H-OCT, higher than standard dose of OCT; 5-HTP, hydroxytryptophan; L-DOPA, L-dihydroxyphenylalanine; L-OCT, standard dose of OCT; MRI, magnetic resonance imaging; OCT, [¹¹¹In]-DTPA-D-Phe-pentetreotide; PET, positron emission tomography; SPECT, single photon emission computed tomography; TE, echo time; TR, repetition time.

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