Somatostatin Receptor Scintigraphy

Neuroendocrine tumors (NET) can be prevented with the help of the Somatostatin Receptor Scintigraphy be diagnosed. A somatostatin analog is radioactively labeled with a tracer and accumulates in high density tissues at somatostatin receptors. The radiation exposure of this examination corresponds roughly to that of a computed tomography of the abdomen.

What is Somatostatin Receptor Scintigraphy?

Somatostatin receptor scintigraphy is a nuclear medicine imaging procedure that can be used to diagnose neuroendocrine tumors (NET) in particular. These express somatostatin receptors in high density to which octreotide, a synthetic somatostatin analogue, binds.

This is radioactively marked and the emitted gamma radiation is detected with a gamma camera. In this way, these tumors, which are often inaccessible to other imaging methods, can be localized. The method is highly sensitive in the diagnosis of neuroendocrine tumors, with the exception of insulinoma.

Function, effect & goals

The main area of ​​application of somatostatin receptor scintigraphy is the diagnosis of neuroendocrine tumors (NET). These are epithelial neoplasms that mainly occur in the abdomen and pancreas. They can be benign or malignant, and have an incidence of 1-2 per 100,000 per year.

These tumors express somatostatin receptors in high density, which is used for nuclear medicine detection. The insulinoma, a tumor originating from the endocrine beta cells (islets of Langerhans) in the pancreas, is the only neuroendocrine tumor that cannot be diagnosed with somatostatin receptor scintigraphy because it does not have such receptors.

The radiopharmaceutical used consists of a somatostatin analog, a strong complexing agent, and a gamma emitter called a tracer. A commonly used somatostatin analog is octreotide, which is why this procedure is also known as octreotide scan. Octreotide is bound to the complexing agent, for example DTPA (diethylenetriaminepentaacetic acid) or DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and radioactively labeled shortly before use.

This happens, for example, with 111 indium, which emits gamma rays and has a half-life of 2.8 days. The compound with DTPA is called 111indium pentetreotide. Because of this short half-life, it is necessary to carry out the radioactive labeling directly before the examination.

The radiopharmaceutical is administered intravenously and is distributed throughout the organism through the bloodstream. The octreotide portion of the molecule binds to somatostatin receptors in the body and accumulates in tissues with high receptor density. These are naturally found in certain brain areas such as the hypothalamus, cortex and brain stem. In addition, various tumors and their metastases express this receptor.

Somatostatin receptor scintigraphy is particularly valuable for the detection of gastroenteropancreatic neuroendocrine tumors (GEP-NET), which can hardly be displayed with other imaging methods. The octreotide scan shows a very high sensitivity here. It is used both for primary diagnosis and for staging (determining the tumor stage) and postoperative control.

Furthermore, the somatostatin receptor scintigraphy is used for the diagnosis of medullary thyroid carcinomas and Merkel cell tumors and for the differential diagnosis of meningiomas versus neuromas. Some breast and colon cancers also express somatostatin receptors. The sensitivity of the octreotide scan is much lower here, which is why it is not used to diagnose these diseases.

The first picture is taken with the gamma camera four hours after the administration of the radiopharmaceutical. The radioactive isotope is now bound to the somatostatin receptors of the organism via the octreotide component and emits gamma radiation when it decays. In areas with a high density of somatostatin receptors, there is increased gamma radiation, which is detected by the gamma camera and displayed as an image.

This is how a tumor can be localized. The examination takes about an hour. It will be repeated the next day. The radiopharmaceutical is excreted through the kidneys and intestines. Alternatives to 111 indium pentetreotide are, for example, 99 technetium tectrotide, with which an even higher sensitivity can be achieved. Other isotopes that can be used are iodine and gallium. The latter is used for positron emission tomography (PET).

You can find your medication here

Risks, side effects & dangers

Gamma rays, like x-rays, are a type of ionizing radiation. These have the ability to remove electrons from atoms, i.e. to ionize them. If molecules of the genome, i.e. DNA, are affected, mutations can occur that can cause cancer.

Such mutations and molecular changes arise again and again in the cells due to various causes. In most cases, however, they can be eliminated by the cellular repair systems.

In the embryonic phase, however, the organism is particularly sensitive to harmful influences. The consequence of radiation exposure in the uterus increases the risk of developing cancer in childhood. For this reason, nuclear medicine examinations are contraindicated in pregnant women. Every patient should avoid intensive contact with pregnant women and small children on the day of the examination.

In children, the indication is strict and the dose of the radiopharmaceutical is reduced according to the age and weight of the child. Since radiopharmaceuticals can accumulate in breast milk, breastfeeding women are recommended to express milk before the examination and to interrupt breastfeeding for a few days after the scintigraphy.

The short half-life of the isotopes used in nuclear medicine studies ensures that the radiation does not remain in the organism for long. The radiation exposure of an octreotide scan in adults is 13-26 mSv (millisievert). This corresponds roughly to the radiation exposure of a computed tomography of the abdomen. For comparison: A simple lung x-ray has 0.02-0.04 mSv. The natural radiation exposure of the environment is 2-3 mSv per year.

Direct side effects are not to be expected, and intolerance reactions to the applied radiopharmaceutical are extremely rare. Patients taking octreotide as a therapeutic agent must stop taking it a few days before the examination.