Sestamibi scan

A sestamibi scan of the parathyroid gland is a nuclear medicine procedure which is performed to identify hyperparathyroidism (or parathyroid adenoma). Sestamibi is a nuclear radiopharmaceutical (methoxy-isobutyl-isonitrile) which is bound to the radioactive isotope Tc99m. Tc99m-sestamibi is taken up by both the thyroid gland and the parathyroid gland, but can also be taken up by enlarged thymus or lymph nodes within the neck, and is also used in nuclear myocardial perfusion imaging and in detection of very early stage breast cancer.

The principle of the procedure is that the Tc99m-sestamibi is absorbed at a greater rate in a hyperfunctioning parathyroid gland than in a normal parathyroid gland. This is dependent on several histologic features within the abnormal parathyroid gland itelf. Sestamibi imaging is correlated with the number and activity of the mitochondria within the parathyroid cells, such that oxyphil parathyroid adenomas have a very high avidity for sestamibi, while chief cell adenomas have some affinity but to a lesser degree, and clear cell parathyroid adenomas have almost no imaging quality at all with sestambi. Some researchers have also attempted to quantitate or characterize the imaging capabilities of parathyroid glands by the MDR gene expression. Approximately 60 percent of parathyroid adenomas may be imaged by sestamibi scanning. The natural distribution of etiologic causation for primary hyperparathyroidism is roughtly 80 % solitary adenomas, 12 % diffuse hyperplasia, 2 % multiple adenomas, and 1 % cancer. In patients with multiglandular parathyroid disease, imaging is not as reliable. In addition, size limitation of the abnormal gland can limit the detection by radionuclide scanning. SPECT (3 dimensional) imaging, as an adjunct to planar methods, can greatly increase the scan sensitivity and localization reliablity, especially in cases of small or parathymic adenomas. By using a gamma camera in nuclear medicine, the radiologist is able to determine if one of the four parathyroid glands is hyperfunctioning, if that is the cause of the hyperparathyroidism. Theoretically, the hyperfunctioning parathyroid gland will take up more of the Tc99m-sestamibi, and will show up 'brighter' than the other normal parathyroid glands on the gamma camera pictures, especially because of the internal biofeedback loop within the body with calcium inherently feeding back to calcium-receptors and inhibiting parathyroid hormone production within the normal parathyroid glands. Sometimes this determination must be made three or four hours later when activity taken up by the thyroid and normal parathyroid glands fade away; the abnormal parathyroid gland retains its activity, while the radiopharmaceutical is eluted out of the normal thyroid gland. However, in patients with nodular goiter or functional tumors of the thyroid gland, increased uptake of the sestamibi agent is possible and make parathyroid localization difficult or confusing.

By knowing which of the four parathyroid glands is hyperfunctioning, a surgeon is able to remove only the one parathyroid gland that is producing excessive amounts of parathyroid hormone and no longer under the biochemical control of the body, and leave the other 3 normal parathyroid glands in place. This operation is now termed a "minimally invasive parathyroidectomy", sometimes utilizing a radionuclear detection probe, and correlated with intra-operative parathyroid hormone level measurements. The remaining 3 glands are able to properly regulate serum calcium levels appropriately after the resolution of the hypercalcemia, as the calcium receptors lead to stimulation of parathyroid hormone secretion.


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