Misdiagnosis of Paraganglioma by 123I-mIBG Without Stable Iodine Blockade of Thyroidal Radioiodine Uptake

Abstract

123I/131I-mIBG scan is an established tool for the localization and treatment of neuroendocrine tumors such as paragangliomas (PGL). To minimize thyroid irradiation by the radioactive iodine in the mIBG preparation, blockade of thyroidal iodine uptake with high doses of stable iodine used to be given routinely as part of all mIBG protocols. As 123I is now more frequently utilized than 131I, concern about thyroid radiation has lessened and thyroid blockade is often considered unnecessary. However, in certain situations, the lack of thyroid blockade can significantly impact treatment decisions. This report describes two patients who had anterior mediastinal masses incidentally discovered on CT scans, and on further evaluation were found to have symptoms suggesting catecholamine excess with mildly elevated plasma normetanephrine levels. 123I-mIBG scans were performed without thyroid blockade, which demonstrated accumulation of tracer in the masses that were therefore deemed positive for PGL. Both patients underwent surgical resection of the masses with their surgical pathology revealing ectopic thyroid tissue (ETT). These cases illustrate that if appropriate thyroid blockade is not performed, ETT concentrating radioiodine from mIBG can lead to falsely positive mIBG scans and unnecessary surgical procedures. We conclude that in the setting of a mass suspicious for PGL in a location potentially representing ETT, such as the mediastinum, thyroid blockade should be employed for mIBG protocols to avoid false positive scans caused by ETT.

Personalized Management of Malignant and Non-Malignant Ectopic Mediastinal Thyroid: A Proposed 10-Item Algorithm Approach

“…The sample-focused analysis according to our methods identified another 37 EMT adults (22 females and 12 males; specific demographic data were not available for three cases; mean age was of 56.32 years, females subgroup: mean age of 56.04, range: 30-80 years; males subgroup: average age of 56.83, range: 31-74 years; across 35 articles featuring a single case study and two articles introducing each two subjects [74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93]), whereas benign EMTs were associated with a normal thyroid panel in terms of function, negative autoimmunity, and lack of nodules/cancer/goiter in eutopic (cervical) gland before and after EMT removal or identification if the EMT was not resected (of note, we also, included the cases whereas no specific thyroid data were provided, thus, it was presumably normal) [13,29, (Table 3). Regardless of the pathological traits, awareness of EMTs is essential, while removal was decided in most of the cases (rather than conservative approach) depending on the location and EMT anatomical features; the risk of malignancy (the rate of conversion from benign to a malignant EMT is not clearly understood, especially in long standing goiter-like EMT); the ectopic tissue enlargement with compressive symptoms/signs such as respiratory obstruction or compression on mediastinal organs; the risk of hemorrhage [84]; the patient's co-morbidities and medical/surgical history as well as the general health status [7,13].…”

“…There are current gaps in EMT management, making it a matter of personalized approach: a low index of suspicion; no specific criteria for providing biopsy and/or FNA, neither for opting in favor of EMT resection in each case; lack of standardized surgical Investigations serving for differential diagnosis of the mediastinal mass such as an ectopic parathyroid tumor causing a primary hyperparathyroidism (blood calcemic and PTH assays) [54,81,115] or a paraganglioma (requiring measurements of blood and 24 h urinary metanephrines and normetanephrines at least twice) [9,76,81,102,105]. Of note, an ectopic (mediastinal) parathyroid tumor is more frequent than an EMT [156,157].…”

“…The largest diameter varied from 1 to 15 cm; no correlation between the size and a malignancy trait in EMTs could be established. The size groups (according to the largest diameter via our practical perspective) may be regarded as follows: <2 cm [22,59,81,89,118]; between ≥2 cm and <3 cm [11,35,55,83]; between >8 cm and <10 cm [13,73,102,117]; more than 12 cm [37,45,60,115]. The most common groups were as follows: between ≥3 cm and <5 cm [9,58,62,80,85,86,93,95,103,106,119]; between ≥5 cm and <6 cm [29,74,78,79,98,100,121,122]; between ≥6 cm and <8 cm [50,55,64,70,75,82,84,88,92,100,…”

Novel [18F]-Labeled Meta-Bromobenzylguanidine Derivatives: Potential Positron Emission Tomography Imaging Probes for the Norepinephrine Transporter

“…26,27 There is higher in vivo deiodization of noncarrier-added [ 123 I]MIBG, resulting in a more pronounced background and the accumulation of radioactivity in the thyroid. 28 As a single photon emission computed tomography imaging agent, [ 123 I]MIBG is also limited by certain drawbacks, such as the lack of an effective quantitative decay correction and lower sensitivity and spatial resolution. Therefore, the development of novel NET-targeting positron emission tomography (PET) imaging agents with optimal pharmacokinetic properties is an urgent and indispensable clinical need.…”

“…The NET is the receptor for NE, which transports approximately 80–90% of the NE from synapses to presynaptic neurons by an active, saturable, and energy-dependent reuptake (uptake-1) process. , Meta-iodobenzylguanidine (MIBG), a structural analogue of NE, shares the same uptake and release mechanism as NE in neuroendocrine tumors and myocardial sympathetic nerve endings. − Thus, radiolabeled meta-iodobenzylguanidines [ 123 I]­MIBG and [ 131 I]­MIBG have been widely used for neuroendocrine tumor imaging and radiotherapy in the clinic, respectively. [ 123 I]­MIBG has also been used to predict the progression of heart failure, arrhythmias, and cardiac death. − However, MIBG is not an ideal imaging agent due to its slower nontarget tissue clearance. , There is higher in vivo deiodization of noncarrier-added [ 123 I]­MIBG, resulting in a more pronounced background and the accumulation of radioactivity in the thyroid . As a single photon emission computed tomography imaging agent, [ 123 I]­MIBG is also limited by certain drawbacks, such as the lack of an effective quantitative decay correction and lower sensitivity and spatial resolution .…”