Protecting the thyroid: I-123 MIBG blockade passes the test
Whether to block or leave unblocked was the question when evaluating I-123 meta-iodobenzylguanidine (MIBG) for thyroid uptake. Patients who received thyroid blockade to inhibit absorption of unbound radioiodine during cardiac imaging showed less overall thyroid activity in planar imaging, according to a study published Jan. 2 in the Journal of Nuclear Medicine.
MIBG, a nerve cell biomarker used in imaging of a variety of diseases from neuroendocrine cancers to heart failure, as was the case in this study, has undergone a switch in radioiodine from the use of I-131 to I-123, generally thought superior in dosimetry evaluations. However, uptake of unbound radioiodine can lead to thyroid damage and subsequent hypothyroidism if not aggressively blocked prior to scanning. This is especially important for young patients. Adult cardiac patients in the Myocardial Imaging for Risk Evaluation in Heart Failure (ADMIRE-HF) drug trial were tested in order to evaluate not only whether a standard blockade was successful, but also how much thyroid activity on scans was actually due to thyroid uptake of free radioiodine or some other cause.
A team of researchers including Nicholas C. Friedman, MD, from the department of nuclear medicine at Edward Hines VA Medical Center in Hines, Ill., evaluated a small group of either blocked or unblocked subjects, an intermediate group and a large cohort of 669 patients to determine the cause of thyroid activity in I-123 MIBG scans and the efficacy of thyroid blockade. Results showed a trend toward significantly less thyroid activity in blocked patients, especially during visual interpretation. The team was also able to show that sympathetic nerve uptake, and not thyroid uptake, accounts for some of the localized activity in I-123 MIBG scintigraphy.
“There are sympathetic nerve fibers that reach the thyroid from the superior, middle, and inferior ganglia of the sympathetic trunk, and these small nerves enter the gland along with the blood vessels,” wrote Friedman, et al. “It is therefore reasonable to presume that a low level of [MIBG uptake] occurs in the presynaptic nerve terminals. The present study provides the first large body of data allowing comparison of the low level of thyroid activity in blocked patients (potentially reflecting specific MIBG uptake) to the higher uptake in the nonblocked cohort.”
In the prospective trial, thyroid blockade consisting of a saturated solution of potassium iodide (SSKI) was presented as an option to patients. A pilot project of 15 patients showed no significant difference in thyroid counts at four hours after injection of radioiodine using a probe evaluation. Another group of 152 patients’ scans were visually scored and scans from yet another cohort of 669 patients, 442 blocked, were quantitatively analyzed. Less thyroid activity was seen by visual scoring in the second cohort and region-of-interest analyses in the third cohort showed significantly more net thyroid activity in unblocked patients.
“On the basis of three different methods for assessing thyroid uptake of I-123, use of thyroid blockade pretreatment in I-123 MIBG imaging prevents increase of thyroid activity over time because of uptake of unbound I-123,” wrote the authors.
Ubiquitous low-level thyroid activity across study models was deemed a probable result of specific sympathetic nerve terminal uptake of the agent.