Gastrointestinal iodine-131-meta-iodobenzylguanidine activity

Radioactivity in the colon during 131-I-meta-iodobenzylguanidine (MIBG) scintigraphy may obscure or be mistaken for tumor uptake. Fecal excretion of radioactivity was examined in eight patients following therapeutic 131-I-MIBG administration (123-218 mCi, 4.551-8.066 GBq) and was found to be 0.02-1.93% of the administered dose. Semiquantitative grading of colonic activity on scintigraphy was inversely related to fecal excretion. An additional patient with marked colonic activity was studied before and after an enema: all visible gut activity was evacuated. We conclude that radioactivity in the colon seen in 131-I-MIBG scintigraphy is due largely to gut excretion of 131-I and is not due to 131-I-MIBG uptake in the autonomic innervation of the gut. Laxatives and enemas are suggested for patients in which such gut radioactivity may lead to difficulties in interpretation.