Usefulness of somatostatin receptor scintigraphy in patients with occult ectopic adrenocorticotropin syndrome

SRIF receptor scintigraphy (SRS) has been proposed for the localization of ectopic AGTH-secreting tumors responsible for Gushing's syndrome. However, in most cases reported, the tumors were also visible using conventional ima ging. Therefore, the usefulness of SRS in localizing truly occult ectopic A CTH-secreting tumors remains unknown. We report the results of SRS in 12 pa tients with ectopic ACTH syndrome (EAS) and in whom the source of ACTH was occult at presentation despite carefully performed conventional imaging. Th e diagnosis of EAS was made by identification of an ACTH-secreting tumor du ring follow-up in 5 patients or given a pituitary-to-peripheral ACTH ratio of 1.9 or less during petrosal sinus sampling combined with CRH injection a nd a negative pituitary magnetic resonance imaging (MRI). Whole-body planar SRS, using In-111-pentetreotide, was performed 19 times in the 12 patients during initial workup and/or follow-up. Axial tomography imaging (single-p hoton emission-computed tomography) was performed in 7 of these. Convention al imaging was performed within a month of SRS, allowing comparison of the two approaches for the localization of the ACTH-secreting tumors. In additi on, the response of plasma cortisol, after a single injection of 200 mu g o ctreotide, was studied in 6 patients. Five patients had negative SRS and conventional imaging studies. The source of ACTH secretion remains occult despite 10-55 months of follow-up in four of these, whereas a 2-cm ileal carcinoid tumor, with liver micrometastases , was found at laparotomy in one patient, 14 months after presentation. SRS was positive in 4 of 12 patients. It was false-positive in 1 patient wi th follicular thyroid adenoma. Nineteen months after presentation, SRS iden tified liver metastasis that was also visible using MRI in one patient, but the primary tumor remains occult. SRS identified a 10-mm pancreatic tumor that became detectable, using computed tomography (CT) scanning 9 months la ter, in 1 patient; and 2 mediastinal lymph nodes of 10 mm, previously ignor ed by MRI, in another patient, whereas no tumor was detectable within the p arenchymal lung. SRS had little influence on therapeutic options in these 2 patients, in whom no final diagnosis could be made. Repetition of SRS duri ng the follow-up of patients with previously negative scintiscans was usele ss. Conventional imaging was positive in 6 of 12 patients. In the 2 patients wi th pancreatic tumor and isolated mediastinal lymph nodes, conventional imag ing studies were interpreted as positive only after the results of SRS. One patient had liver metastasis that was also visible using SRS. Thin-section CT scanning visualized ACTH-secreting bronchial tumors and metastatic medi astinal lymph nodes of 10-15 mm in diameter in 3 patients after 14-72 month s of followup, whereas SRS was negative. There was no evident relationship between the endocrine status (hyper- or e ucortisolism) and the results of SRS. The in vivo response of plasma cortis ol to octreotide correlated to the results of SRS in 4 of 6 cases. In concl usion, both imaging procedures had a low diagnostic yield in this series. H owever, the sensitivity of SRS for the detection of bronchial carcinoids wa s lower than that of thin-section CT scanning. We therefore advocate the us e of conventional imaging, including thin-section CT scanning of the chest, analyzed by experienced radiologists, as the first-line investigation in p atients with occult EAS. SRS should not be repeated during the follow-up in patients with a previously negative scintigram.