Iodine-131-MIBG scintigraphy in adults: interpretation revisited?

Iodine-131-metaiodobenzylguanidine (MIBG) scintigraphy is a reliable method 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 131I-MIBG. We recently observed an increase in the frequency of visualization of the adrenal medulla in patients injected with 74 MBq 131I-MIBG. Therefore, we retrospectively evaluated the pattern of uptake and potential changes between 1984 and 1994.

Scintigraphic data from 103 patients referred for suspected pheochromocytoma were reviewed randomly. Data from 19 patients with medullary thyroid carcinoma were analyzed separately. Patients were injected with 74 MBq 131I-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 (most intense activity in the picture). Semiquantitative indicies were evaluated for discriminating between normal adrenal medullae and pheochromocytomas. Twenty-seven pheochromocytomas were surgically proven in 25 patients.

A visual score > or =3 was noted in 81% and 90% of the pheochromocytomas at 24 hr and 48 hr postinjection, respectively. From 1984 to 1988, 16% and 31% of adrenal medullae were seen at 24 and 48 hr postinjection, respectively, whereas from 1989 to 1994, 56% and 73% were visualized at 24 and 48 hr postinjection, respectively. Before 1989, the best cutoff criterion to identify a pheochromocytoma, determined from receiver operating characteristic curve analysis, was a score > or =1 at 24 hr and > or =3 at 48 hr postinjection, with a sensitivity and specificity of 92% and 84% at 24 hr and 92% and 99% at 48 hr postinjection. From 1989, the best cutoff was a score > or =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 ratios were the best discriminators between normal and pathological adrenals. They were, however, of little use because of the overlap between normal adrenal medullae and pheochromocytomas.

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 years, however, this rate has been increasing, possibly because of the increased specific activity of 31I-MIBG. Adequate interpretation should take into account that a faint or definite uptake may be visible in more than 50% of normal adrenal medullae.