Abstract
Metaiodobenzylguanidine (MIBG) is actively concentrated in adrenergic neuroendocrine tissues and tumors by an active, energyand sodium‐dependent, high‐affinity, saturable mechanism. This has proved successful, when labeled with 131‐l or 123‐l, in scintigraphically depicting pheochromocytomas and neuroblastomas. For imaging purposes 123‐l has multiple advantages over 131‐l; the gamma ray energy is ideal for modern instruments the decay by electron capture limits the particulate emissions, and the short half‐life reduces the radiation burden. It is thus possible to use doses of 123‐I‐MIBC 20 times as large as doses of 131‐I‐MIBG with equivalent absorbed radiation doses. Disadvantages of 123‐I include the cost and difficulties in the regular delivery of this short‐lived radionuclide. For most imaging purposes 123‐I‐MIBC is the optimal agent if logistical problems in its sup ply can be overcome.