A Scintigraphic Comparison of Iodine-123-metaiodobenzylguanidine and an Iodine-labeled Somatostatin Analog (Tyr-3-octreotide) in Metastatic Carcinoid Tumors

A number of neoplasms are known to express somatostatin receptors, and the use of somatostatin receptor imaging in their localization has recently been described. We compared an 123I-labeled somatostatin analog Tyr-3-octreotide (TOCT) and 123I-labeled metaiodobenzylguanidine (MIBG) scintigraphy in seven patients with histologically proven metastatic carcinoid tumors. The optimum time for identifying tumor uptake on scanning after [123I]MIBG was 24-48 hr, and after 123I-TOCT 10-30 min postinjection. Both radiopharmaceuticals showed a varying spectrum of tracer uptake ([123I]MIBG showed no uptake in one patient; minimal in two; moderate in two; and intense in two; 123I-TOCT showed no uptake in two patients; minimal uptake in one; moderate uptake in two; and intense uptake in two). In two patients, 123I-TOCT identified metastatic lesions not seen by [123I]MIBG scintigraphy. These preliminary results suggest that [123I]MIBG and 123I-TOCT are useful and complementary imaging techniques for detecting metastatic carcinoid tumors.