Parathyroid hyperplasia (Tc-99m sestamibi parathyroid scintigraphy)
Presentation
Chronic renal failure on dialysis. Elevated parathyroid hormone (PTH) and serum calcium.
Patient Data
Parathyroid scintigraphy
Parathyroid scan performed using 20 mCi of Tc-99m sestamibi IV.
Early phase images (15 min post injection) demonstrate uptake in the central neck in the region of the thyroid. While no ectopic foci are visible, the uptake in the region of the thyroid is abnormally heterogeneous. This suggests the possibility of superimposed abnormal parathyroid glands and/or uptake of radiotracer by thyroid nodules.
Delayed phase images (2 hours post injection) demonstrate normal washout of the thyroid gland with 4 foci of radiotracer retention in the regions of the parathyroid glands. Findings were favored to represent parathyroid hyperplasia and less likely multiple parathyroid and/or thyroid adenomas.
Pathology final diagnosis:
a. soft tissue, left upper parathyroid, biopsy: compatible with parathyroid hyperplasia (see comment)
b. soft tissue, left lower parathyroid, biopsy: compatible with parathyroid hyperplasia (see comment)
c. soft tissue, right lower parathyroid, biopsy: compatible with parathyroid hyperplasia (see comment)
d. soft tissue, right upper parathyroid, biopsy: compatible with parathyroid hyperplasia (see comment)
Comment: Histologic sections demonstrate benign parathyroid parenchyma remarkable for a nodular proliferation of predominantly chief cells, although there is a multinodular proliferation of variable numbers of oncocytic and transitional oncocytic cells as well as clear cells. Stromal fat is markedly reduced. Histologic features worrisome for malignancy are not evident. These histologic features are indistinguishable from a parathyroid adenoma, however the vast majority of adenomas involve a single gland. These histologic features are compatible with a history of secondary hyperparathyroidism.
Case Discussion
Hyperparathyroidism is the result of a solitary parathyroid adenoma in the vast majority of patients (~85%). Infrequently, hyperparathyroidism can be due to multiple adenomas. Parathyroid hyperplasia, as seen in this patient, accounts for the remaining cases, with parathyroid carcinoma encountered in less than 1% of instances.
Dual-phase imaging with Tc-99m sestamibi is frequently used to localize parathyroid adenomas for surgical planning. For the exam, images of the neck and chest are performed at 15 minutes and 2-4 hours post-injection. SPECT/CT imaging can also be performed and has been shown to provide superior diagnostic performance and more accurate lesion localization.
Tc-99m sestamibi normally gets taken up by the parathyroids as well as the salivary glands, thyroid, and heart. Normal parathyroid glands, due to their small size, are not seen on exam. Parathyroid adenomas however, demonstrate increased uptake on the early images and abnormal retention on delayed images, while normal organs such as the thyroid show washout of the radiotracer. A significant percentage of adenomas though may not show typical retention and therefore close attention should be paid to any abnormal foci of uptake on early images if SPECT/CT is not performed.
Given that the size of the abnormal parathyroid gland is critical to its scintigraphic detection, it is understandable that the study is much less sensitive for the detection of parathyroid hyperplasia where each gland is only mildly increased in size and hence usually only used if hyperparathyroidism persists after initial surgery.
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