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.
Methods: 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.
Results: 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.
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 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.