IODINE-131-MIBG SCINTIGRAPHY IN ADULTS - INTERPRETATION REVISITED

Iodine-131-metaiodobenzylguanidine (MIBG) scintigraphy is a reliable m ethod used to diagnose pheochromocytoma. Although the adrenal medulla usually is not visualized, faint uptake can be observed in 16% of the patients 48-72 hr after injection of 18.5-37 MBq I-131-MIBG. We recent ly observed an increase in the frequency of visualization of the adren al medulla in patients injected with 74 MBq I-131-MIBG. Therefore, we retrospectively evaluated the pattern of uptake and potential changes between 1984 and 1994. Methods: Scintigraphic data from 103 patients r eferred for suspected pheochromocytoma were reviewed randomly. Data fr om 19 patients with medullary thyroid carcinoma were analyzed separate ly. Patients were injected with 74 MBq I-131-MIBG and imaged at 24 hr postinjection, 48 hr postinjection, or both. Adrenal uptake was scored visually as 0 (no visible uptake) and 1 (uptake just visible) to 4 (m ost intense activity in the picture). Semiquantitative indicies were e valuated for discriminating between normal adrenal medullae and pheoch romocytomas. Twenty-seven pheochromocytomas were surgically proven in 25 patients. Results: A visual score greater than or equal to 3 was no ted in 81% and 90% of the pheochromocytomas at 24 hr and 48 hr postinj ection, respectively. From 1984 to 1988, 16% and 31% of adrenal medull ae were seen at 24 and 48 hr postinjection, respectively, whereas from 1989 to 1994, 56% and 73% were Visualized at 24 and 48 hr postinjecti on, respectively. Before 1989, the best cutoff criterion to identify a pheochromocytoma, determined from receiver operating characteristic c urve analysis, was a score greater than or equal to 1 at 24 hr and gre ater than or equal to 3 at 48 hr postinjection, with a sensitivity and specificity of 92% and 84% at 24 hr and 92% and 99% at 48 hr postinje ction. From 1989, the best cutoff was a score greater than or equal to 3 at both imaging sessions, with a sensitivity and specificity of 82% and 100% at 24 hr and 100% and 97% at 48 hr postinjection. Among the semiquantitative indicies, the adrenal-to-liver and adrenal-to-heart r atios were the best discriminators between normal and pathological adr enals. They were, however, of little use because of the overlap betwee n normal adrenal medullae and pheochromocytomas. Conclusion: The high rate of visualization of the normal adrenal medulla in this study was related to the larger-than-usual injected dose (74 MBq). Over recent y ears, however, this rate has been increasing, possibly because of the increased specific activity of I-131-MIBG. Adequate interpretation sho uld take into account that a faint or definite uptake may be visible i n more than 50% of normal adrenal medullae.