Summary, conclusions, and future directions of [131I]metaiodobenzylguanidine therapy in the treatment of neural crest tumors.

The role of diagnostic [131I/123I]metaiodobenzylguanidine (*I-MIBG) scintigraphy in the management of pheochromocytoma and neuroblastoma is established, but for other neural crest tumors is less defined. Radiopharmaceutical therapy of all these tumors with large activities of suitably radiolabeled MIBG is a compelling concept. In the five years since the first workshop on 131I-MIBG therapy held in Rome, the initial therapeutic promise appears to have been maintained for neuroblastoma and pheochromocytoma. A significant fraction of patients enter partial remission but complete remission is rare and relapse frequent. To date, experience with other neuroendocrine tumors and the use of 125I in place of 131I remains limited. Many promising areas remain incompletely explored. These include development of appropriate in vitro cultures and animal models, basic pharmacological mechanisms, drug interactions, macro- and microdosimetry and human clinical trials. The latter includes determining dose-limiting toxicity of 131I- and 125I-MIBG, treatment of patients at earlier times or stages of disease, optimal integration with other therapy including granulocyte-stimulating factor and marrow transplant rescue from otherwise limiting myelotoxicity. Progress to date has been slow and painstaking, but nevertheless significant, while the future holds both challenges and promise.