Intraoperative radionuclear 125I-labeled metaiodobenzylguanidine scanning of pheochromocytomas and metastases.

We developed a technique to assess the feasibility of intraoperative radionuclear detection of pheochromocytomas and their metastases. Thirteen patients were entered into the study: five control subjects with nonchromaffin adrenal tumors, eight with pheochromocytomas, and one of these patients showing bone metastasis. Each subject received thyroid blockade and an intravenous injection of 500 microCi (37 megabecquerels) 125I-labeled metaiodobenzylguanidine (MIBG) 3 days before surgery. In the five control subjects, adrenal tumor uptake never exceeded the liver or spleen uptake. One patient with a negative preoperative MIBG scan demonstrated no intraoperative uptake. Five patients with pheochromocytoma had positive preoperative scans and in one other patient preoperative scanning was not done. In each of these six patients intraoperative count ratio of pheochromocytoma/liver from 14:2 to 250:16 and pheochromocytoma/contralateral adrenal ratio from 60:1.5 to 60:16 was demonstrated. An intraoperative scan in one of these patients detected two small metastatic tumor deposits previously overlooked by the surgeon after removing a larger mass that had been localized by a preoperative 131I-MIBG scan. A negative preoperative scan in one patient was followed by an intraoperative scan demonstrating a bone metastasis with a ratio of metastasis/normal bone of 10:0.5. Specimen studies demonstrated a significant MIBG uptake ratio of tumor/plasma ranging from 95 to 667 (average 404 +/- 242) greater than in control subjects (average 25 +/- 41); in the patient with metastasis the uptake ratio of metastasis/normal bone reached 98.4. We conclude that intraoperative 125I-MIBG scanning might detect pheochromocytoma deposits overlooked by preoperative 131I-MIBG scans.