Radioiodinated metaiodobenzylguanidine in the diagnosis and therapy of carcinoid tumors.

Carcinoid tumors account for less than 1% of all malignancies and the majority arises in the gastrointestinal system. These tumors are slow-growing compared with adenocarcinomas and they differ from the other neuroendocrine malignancies by their protean clinical presentation. Carcinoid tumors were previously considered indolent, but they can manifest malignant characteristics with metastatic spread which often results in a poor prognosis. Although there have been advances in diagnostic and treatment modalities, carcinoid tumors are still frequently diagnosed late, often when the tumor has metastasized and patients have developed carcinoid syndrome. Diagnosis, prognosis and treatment options are based on biochemical markers and imaging investigations. High concentration of urinary 5-HIAA, elevated plasma serotonin and chromogranin A levels help to establish the initial diagnosis of carcinoid tumors. In addition to the computed tomography and magnetic resonance imaging, molecular imaging modalities such as OctreoScan, metaiodobenzylguanidine (MIBG) imaging and more recently PET imaging are used in detecting the primary malignancy and metastatic involvement. Surgery is the mainstay of treatment of non-metastatic carcinoid tumors. Cytotoxic chemotherapy has limited role because of the chemoresistant nature of these tumors. Because carcinoid tumors express somatostatin receptors, somatostatin analogues, which inhibit release of serotonin and other neuroendocrine peptides, are often used, but their use is limited to symptom control. Treatment using high doses of radionuclides, such as radiolabeled somatostatin analogues and MIBG, is a more recent option, which offers a definite advantage in management. In this article, we review the current state of the art in the diagnosis and treatment of carcinoid tumors as well as the role of MIBG in their diagnosis and management.