Radiolabeled mataiodobenzylguanidine in the diagnosis of neural crest tumors

Due to its diagnostic application, consideration was given to the therapeutic potential of 131I-MIBG in neural crest tumours, mainly in malignant pheochromocytomas, paragangliomas, neuroblastomas (NB), carcinoids and medullary thyroid carcinomas (MTC). The therapeutic procedure consists of a) thyroid blockade; b) administration of high specific activity 131I-MIBG; c) single doses, varying from 3.7 to 9.5 GBq, given by slow i.v. infusion (2-3 hours); d)monitoring of the patient during the infusion of the tracer. Targeted radiotherapy with 131I-MIBG in malignant pheochromocytomas, paragangliomas, carcinoids and medullary thyroid carcinomas, was shown to be effective with partial reduction of tumoral lesions (mainly in pheochromocytomas with 58% of objective responses) and palliation in metastatic tumors; in a few pheochromocytomas also succeed in eradicating the residual/recurrent tumor. In patients with stage IV NB who failed to respond to or relapsed after conventional chemotherapy, MIBG therapy showed an important palliative role. A significant therapeutic improvement in the outcome of stage III and IV NB patients was obtained by introducing 131I-MIBG as a first line therapy, partial or even complete responses occurring in more than 60% of the cases treated. Experiences combining chemotherapeutic agents and 131I-MIBG are also in progress with encouraging results. MIBG therapy is well tolerated; toxicity is limited to minor hematologic toxicity and patients generally recover spontaneously. The risk of pancytopenia rises in patients with bone and/or bone marrow metastases; in these cases bone marrow harvesting is recommended. An alternative approach to 131I-MIBG therapy in MTC uses radiolabeled monoclonal antibodies. A novel immunotargeting method, which includes a bispecific antibody and 131I as a radiolabel, seems to be very promising.