Diagnosis of adrenal tumors with radionuclide imaging

The development of radiolabeled cholesterols in 1969 as precursors of adrenocortical steroid production allowed the first noninvasive imaging of the adrenal cortices. FDA-NDA approval in 1984 should allow routine use of these agents in most hospitals. NP-59 is most commonly used in the diagnosis and management of Cushing syndrome; the second most common use is in the diagnosis of primary aldosteronism. It is also helpful in the differential diagnosis of adrenal and ovarian hyperandrogenism and hirsutism, and is the only noninvasive method of detecting unilateral adrenocortical hypofunction. The newest and most popular use is in the differential diagnosis of asymptomatic masses in the region of the adrenal gland discovered incidentally with CT scan ("incidentalomas"). In this situation, the NP-59 scan can define whether the tumor is in the adrenal gland and if it is functional or nonfunctional. We believe that, in the future, radiolabeled enzyme inhibitors might offer better diagnostic imaging of the adrenal cortex, although these agents will probably not be available for routine use for some time. Our development of a radioiodinated guanethidine analog, 131I-MIBG, has allowed differentiation of normal adrenal medullary function from bilateral adrenal medullary hyperplasia before the development of hypertension or tachycardia, diagnostic increases in plasma or urinary catecholamines, or abnormal CT scans. The search for a pheochromocytoma should begin with 131I-MIBG scintigraphy. While over 90% of primary pheochromocytomas occur in the abdomen, neither a survey of the abdomen nor the finding of a single tumor should conclude the search. Whereas the CT scan can only detect a mass, the MIBG scan not only detects the mass but proves whether the mass is a pheochromocytoma. We have detected small or recurrent pheos when all other localizing studies were normal. Present drug and X-ray therapy is relatively ineffective in treating metastatic cancer of the adrenal medulla. Preliminary results in five patients indicate that, for the first time, a complex organic molecule can carry 131I into adrenal neoplasms reproducibly and in therapeutically effective doses of irradiation. We may also be able to select which patients will respond to therapeutic doses of 131I-MIBG and which patients will not respond.