A sestamibi scan is a highly sensitive and highly specific nuclear medicine test used to locate and image an overactive parathyroid gland in a patient with known hyperparathyroidism. Information from the
test can help with planning for surgery to remove the overactive gland.
Located in the neck behind the thyroid gland, the four parathyroid glands are pea-sized endocrine glands that are responsible for the production of parthyroid hormone or PTH. PTH is important in the balance of calcium and phosphate throughout the body.
Under normal conditions, low calcium concentrations in the bloodstream prompt the parathyroid gland to put out increased amounts of PTH. PTH acts on several areas of the body. It directs the kidneys to absorb calcium back into the body, rather than flushing it out of the body in the urine. It activates osteoclasts in bone to degrade bone material, releasing calcium for use in the body. It increases the activity of vitamin D, which allows more calcium to be absorbed in the intestine.
Hyperparathyroidism is usually due to the presence of an adenoma, a benign (not cancerous) growth on one or more of the parathyroid glands. A sestamibi scan is used to generate images of the parathyroid glands prior to surgery so that the surgeon knows which of the four glands will require removal. Surgery to remove a parathyroid gland is called a parathyroidectomy .
During a sestamibi scan, the patient is given an injection of the radioactive material technetium-99, bound to a tiny protein called sestamibi. Unlike normal parathyroid glands, adenomatous parthyroid glands absorb the radioactive material, permitting visualization and localization of the tumor or tumors on the scan images. This test can be performed in preparation for an operation to remove the parathyroid adenoma, or during the course of such an operation (intraoperatively).
Hyperparathyroidism is a condition in which one or more of the parathyroid glands become overactive. Too much bone is broken down, and too much calcium circulates in the bloodstream (termed hypercalcemia). The consequences of this excess bone breakdown and excess circulating calcium include:
- Decreased appetite
- Nausea and vomiting
- High blood pressure
- Difficulty thinking
- Poor memory
- Excess thirst
- Frequent urination
- Thinner, weaker bones
- Increased risk of bone fracture
- Kidney stones
Hyperparathyroidism is often considered idiopathic, which means that there is no known underlying cause of the disorder. In about 5% of people with parathyroidism, there is a family tendency for the disorder, such as Familial multiple endocrine neoplasia type 1 or familial hypocalciuric hypercalcemia.
About 100,000 people in the United States are diagnosed with hyperparathyroidism annually. Women are twice as likely to get the disorder than men, and it is more common in people over the age of 60.
Prior to starting the scanner for a sestamibi scan, radioactive contrast is injected through an IV in the patient’s arm. The radionuclide (the technetium-99 bound to setamibi molecules) circulates in the blood stream, concentrating in diseased parathyroid glands. The patient lies on an examination table, and a gamma camera is positioned over the patient’s neck. The camera consists of a crystal detector that detects emitted radiation from the radioactive contrast. A computer converts the signal into a digital image of the parathyroid glands. Scanning is done immediately after injection of the radionuclide, and 1 1/2 to 2 hours after injection. Each scan takes about 10 minutes.
There is nothing patients need to do in preparation for a sestamibi scan. To avoid confusing results, patients who have recently had another type of nuclear scan may need to wait several days to allow that radioactive tracer to leave their bodies, prior to undergoing a sestamibi scan.
Women who are pregnant or who think they may be pregnant are advised against undergoing a sesta-mibi scan. Women who are breastfeeding and who require a sestamibi scan should feed their baby with formula for two days following the procedure, and should pump and discard their breast milk, since it will be contaminated with the radioactive dye.
Adenoma— A benign tumor of an endocrine gland. Hypercalcemia— Excess concentration of calcium in the blood.
Hyperparathyroidism— A condition in which the parathyroid gland is overactive; usually caused by the presence of an adenoma on one or more of the glands.
Parathyroidectomy— An operation performed in order to remove one or more parathyroid gland.
Thyroid gland— An endocrine organ in the neck which produces thyroid hormone. Thyroid hormone is involved in important growth and metabolic processes throughout the body.
There is no aftercare necessary following a sesta-mibi scan. The patient can return immediately to a normal diet and normal activities.
A sestamibi scan poses very little risk to the patient. Rarely, a patient may have an allergy to the radioactive contrast utilized.
Normal results of a sestamibi scan would reveal no uptake of the radionuclide tracer in the neck, suggesting that no parathyroid adenoma is present.
An abnormal sestamibi scan will reveal an area where the radionuclide has been absorbed by a parahyroid adenoma. Even small, single adenomas on a parathyroid gland will “light up,” due to their tendency to absorb the radionuclide. This allows highly accurate localization of the exact area requiring operation. In some cases, a falsely positive sestamibi scan may occur in patients with thyroid disease.
Grainger, R. G., et al. Grainger & Allison’s Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. Philadelphia: Saunders, 2001.
Kronenberg, H. M., S. Melmed, K. S. Polonsy, P. R. Larsen. Williams Textbook of Endocrinology. 11th ed. Philadelphia: Saunders Elsevier, 2008.
Mettler, F. A. Essentials of Radiology. 2nd ed. Philadelphia: Saunders, 2005.
Norton, K. S., et al. “The sestamibi scan as a preoperative screening tool.” American Surgery 68(September 2002): 812–815.
Rosalyn Carson-DeWitt, MD
Seton glaucoma surgery seeTube-shunt surgery