[Nuclear medicine diagnosis of pheochromocytoma with metaiodobenzylguanidine].
Excess secretion of any of the adrenal cortical or medullary hormones contributes to a number of well-known clinical syndromes.. They may result from benign or malignant adrenal tumours, adrenal hyperplasia or, least frequently, from extra-adrenal disease. Differentiation among these possibilities is often impossible on clinical or biochemical grounds alone. Location of the site(s) of excess hormone production in the past depended on relatively insensitive or invasive radiological methods. The non-invasive evaluation began with X-ray computed tomography but the functional significance of anatomical abnormalities cannot be determined from CT scan. Incorporation of specific radiopharmaceuticals into the abnormal tissues allows scintigraphic localization of functional abnormalities with a high degree of efficacy. The combination of adrenal scintigraphy and kompjuterizovanom tomografijom CT or magnetskom rezonancijom MRI should in most cases obviatc the need for more invasive procedures. Phaeochromocytoma is rare in hypertensive population, affecting only an estimated of 0.1%. However, a high index of suspicion is essential, since these tumours have potentially life-threatening cardiovascular effects and their successful resection is curative. Important clinical clues include the presence of orthostatic hypotension in an untreated hypertensive, resistance of hypertension to standard therapy (including possible exacerbation by (beta-blockers). In most cases, the diagnosis can be established by demonstrating high levels of free catecholamines and their metabolites (metanephrines and Vanillylmandelic acid). Clonidine test may be important in some cases. The purpose of this study is to point that metaiodobenzylguanidine (mlBG) has proved to be a safe, sensitive and highly specific agent for the location of phaeochromocytoma. The first successful schinigraphic demonstration of phaeochromocytomas in man was reported in 1981, using a new radiopharmaceutical, 131l-metaiodobenzylguanidinc (mlBG). mlBG is an aralkyl-guanidine which structurally resembles noradrenaline sufficiently to be recognized and be stored in the catecholamine storage vesicles. Whereas unstored noradrenaline is rapidly degraded, the halogenated benzyl ring of mlBG conlers resistance to catechol-o-methyltransferase (COMT) while its guanidino side-chain is resistant to monoamine oxidase (MAO). Uptake of mIBG is inhibited by some inhibitors (reserpine, tricyclic antidepressants, cocaine, labetalol, calcium-chanel blockers...). 131I-mlBG is normally taken up by liver, spleen, myocardium and salivary glands. Thyroid uptake ol liberated radioiodide will also occur unless the thyroid is blocked with stable iodide. The normal adrenal glands are usually not seen but faint uptake may be visible 48-72 h after injection in up to 16% of cases. Hepatic uptake is maximal at 24 h, declining to very low levels by 72 h (even more rapid in patients with phaeochromocytoma. Dosimetric corlsiderations limit the amount of 131l-mlBG that is administered for diagnostic studies. This, coupled with the low detection efficiency of gamma cameras for the 364 keV photon of 131l, led to the introduction of 131l-mlBG as an adrenomedullary scintigraphic agent of choice. In our department we started with mIBG scintigraphy in 1985 and we treated near 1000 patients. In this study we are talking about 180 patients from the beginning of 1996 to the end of 2001 all treated with 131l-mlBG. Like the other worldwide experience with this agent our sensitivity was 88.58% and specificity of 98.46%. Positive predictive value was 88.5% and negative predictive value was 93.46%. False negative results were 6.52% and there were no false positive results. After all we can say that mlBG has proved to be a safe, sensitive and highly specific agent for the location of phaeochromocytoma and neuroblastoma. Other radiolabelled aralkylamines have been examined as potential adrenal medullary scintigraphic agents. None has demonstrated superiority over mlBG in animal or limited human studies. 131l-mlBG should always be considered the radiopharmaceutical of choice for imaging purposes if it is available. 131l-mlBG in high doses is successfully used in therapy of malignant phaeochromocytoma and especially in nuroblastoma.
Pucar
Dragan
D
Department of Nuclear Medicine, Military Medical Academy, Belgrade.
Marković
Stevan
S
D019797:3-Iodobenzylguanidine; D000310:Adrenal Gland Neoplasms / Q000000981:diagnostic imaging*; D006801:Humans; D010673:Pheochromocytoma / Q000000981:diagnostic imaging*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals
D019275:Radiopharmaceuticals; D019797:3-Iodobenzylguanidine