Detection of Ectopic Mediastinal Parathyroid Adenoma on Myocardial Perfusion Scintigraphy with 99mTc-Tetrofosmin

Introduction

Parathyroid ectopia has an incidence up to 25% and is caused by abnormal embryological migration of one (and rarely of more) of the four glands during the early stages of the development.[12] The ectopic gland is located near the tissues with common embryological origin. The parathyroid glands are derived, embryologically, from the endoderm of the third and fourth branchial pouches. More specifically, the inferior parathyroids originate, in common with the thymus, from the third branchial pouch, whereas the superior ones from the fourth branchial pouch. It has been reported that ectopic inferior parathyroids are more common than ectopic superior ones.[12] The most common ectopic site of the inferior glands is in the neck, whereas excessive migration results in low ectopia in the mediastinum. The reported incidence of mediastinal ectopia ranges between 1% and 2%. Ectopic parathyroid adenomas account for 6%–16% of cases of primary hyperparathyroidism (PHP) due to adenomas.[34]

99m Technetium (99mTc) labeled agents such as tetrofosmin and sestamibi are nonspecific tracers used for myocardial perfusion imaging, as well as for benign or malignant diseases.[56]

We present the rare case of ectopic mediastinal parathyroid adenoma detected on myocardial perfusion scintigraphy with 99mTc-tetrofosmin.

Case Report

A 64-year-old woman with chest pain was referred by a cardiologist to undergo a myocardial perfusion single photon-emission computed (SPECT) imaging with 99mTc-tetrofosmin and adenosine pharmacological stress. She had a history of hypertension and dyslipidemia. SPECT images revealed a focus of increased 99mTc-tetrofosmin uptake in the mediastinum [Figure 1]. Due to a suspicion of a possible malignant condition, the patient underwent further imaging with magnetic resonance imaging which showed a well-defined, low-signal, and solid mass in the superior mediastinum, 1.8 cm × 2.1 cm in diameter, compatible with either ectopic thyroid or parathyroid tissue, or enlarged lymph nodes or abnormal thymus, as well as thoracic vertebral collapse, and osteopenia [Figure 2]. Biochemical tests showed increased parathormone levels (92.5 pg/mL), serum calcium 9.5 mg/dL (normal range 8.8–10.4 mg/dL), and normal renal function.

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Figure 1 A focus of increased 99m technetium-tetrofosmin uptake in the mediastinum on myocardial perfusion stress Single Photon Emission Computed Tomography imaging

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Figure 2 Axial T2-weighted magnetic resonance image at the level of the superior mediastinum. A well - defined solid, low- signal 1,8 cm × 2,1 cm mass (arrow) is shown abutting the aortic arch. The differential diagnosis includes enlarged lymph nodes, abnormal thymus, ectopic thyroid and ectopic parathyroid tissue

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The patient underwent a dual phase, dual tracer scintigraphy with 99mTc-sestamibi and 99mTc-pertechnetate which showed a focus of increased uptake and delayed retention of 99mTc-sestamibi in the mediastinum, suggestive of ectopic parathyroid adenoma [Figure 3].

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Figure 3 Dual phase, dual tracer parathyroid scintigraphy with 99m technetium (99mTc)-sestamibi and 99mTc-pertechnetate. A focus of increased uptake and delayed retention of 99mTc-sestamibi is seen in the mediastinum, suggestive of ectopic parathyroid adenoma

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Discussion

The prevalence of normal ectopic parathyroid glands maybe underestimated, and parathyroid ectopias are usually identified in the cases of hyperfunctioning, in patients presenting with hyperparathyroidism. The incidence of PHP due to ectopic parathyroid glands ranges between 10% and 22%, while the clinical manifestation of mediastinal ones is reported to be more severe.[34]

Dual-phase 99mTc-sestamibi scintigraphy is the imaging modality of choice in the investigation of PHP, for the detection of hyperfunctional parathyroid glands, with sensitivity and specificity of 73% and 80%, respectively,[56] while the reported sensitivity and positive predictive value of 99mTc-sestamibi for the localization of ectopic parathyroid adenomas is 89% and 90%, respectively.[7] The characteristic scintigraphic pattern suggestive for the presence of hyperfunctional parathyroid adenoma is the rapid washout of the tracer from the thyroid gland and the retention in the mitochondria-rich oxyphil parathyroid cells.[56] 99mTc-sestamibi scintigraphy is usually combined with 99mTc-pertechnetate scan to avoid false interpretation of 99mTc-sestamibi uptake in thyroid lesions.[56]

Incidental extracardiac 99mTc-tetrofosmin uptake has been reported in benign and malignant conditions including parathyroid adenomas.[8] However, tetrofosmin is not the ideal tracer for parathyroid imaging because it lacks the unique characteristic of differential wash out of sestamibi, and therefore, it cannot reliably distinguish hyperfunctional parathyroid glands uptake from thyroid normal uptake.[56]

Conclusion

The diagnosis of ectopic mediastinal parathyroid adenomas is challenging since they are rare, as in our case, representing a small proportion of all ectopic parathyroid adenomas. Therefore, the early detection and confirmation of this condition are crucial for best patient management. Nuclear medicine imaging provides valuable information about the function of the organs or systems of the human body and every nuclear medicine study should be examined carefully for any unusual or incidental findings which would be very important for patients’ health.