Diagnosis of a Case of Ectopic Parathyroid Adenoma on the Early Image of ^99mTc-MIBI Scintigram

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We report the case of a 64-year-old woman who had a severe hypercalcemia. Serum calcium, intact parathyroid hormone (PTH), 1α, 25 (OH)₂ vitamin D₃ levels were all elevated, and serum phosphorus level was decreased, which were all consistent with primary hyperparathyroidism (PHPT). ²⁰¹Tl/^99mTc subtraction scintigraphy failed to detect any abnormal accumulation in the neck and chest, while ^99mTc-MIBI scintigraphy demonstrated the focal accumulation of increased radiotracer uptake in the mediastinum only on the early image, but not on the delayed image. Neck and chest computerized tomography scanning showed a small nodule at the retrosternal region, and a selective venous sampling study of the intact PTH suggested PTH production from the nodule. Together with the observation of the early image of ^99mTc-MIBI scintigraphy, it was diagnosed that the patient had an ectopic parathyroid adenoma. Video-assisted thoracic surgery was performed. A 15-mm diameter mass, visualized by an intravenous infusion of methylene blue, was excited. The histopathology was consistent with the parathyroid adenoma. The adenoma was composed of mainly chief cells and rarely oxyphil cells. The absence of oxyphil cells would explain the lack of ^99mTc-MIBI retention on late-phase imaging in our case. Even without uptake on the delayed image of ^99mTc-MIBI scintigram, the early image was available for the localization of an ectopic parathyroid adenoma.