How GEP NE Tumors Are Diagnosed (Diagnosis)

If a person has symptoms that can be caused by a GEP NE tumor, the doctor will carry out blood and urine tests to determine whether such a tumor may be present. These tests measure the amounts of certain hormones and other substances associated with GEP NE tumors. Abnormal levels of a particular hormone can help point to the presence of a specific type of GEP NE tumor.

Carcinoid tumors, for example, release large amounts of serotonin. This hormone breaks down into smaller chemicals. One of these chemical byproducts is 5-hydroxyindoleacetic acid (5-HIAA), which can be detected in urine. The laboratory test used most frequently to detect carcinoid tumors measures the amount of 5-HIAA in the urine over a 24-hour period. The 5-HIAA test, however, does not always pick up carcinoid tumors, particularly if they are in the lungs, stomach, or rectum.

Another substance that can indicate the presence of a carcinoid tumor-as well as other types of neuroendocrine tumors-is chromogranin A (CgA). CgA levels in the blood can be measured to detect the presence of a GEP NE tumor. Because CgA can also be elevated in a variety of other conditions, it is not specific to neuroendocrine tumors and is not generally used as either a screening or a diagnostic test.

To determine where the tumor is, how large it is, and whether it has spread, the doctor relies on a variety of tools.

• Endoscopy. This procedure involves using an endoscope-a thin tube with a light on the end. This instrument can be used to locate where a tumor is in the gastrointestinal tract. Endoscopy is most commonly used to detect gastric or rectal carcinoid tumors. Because the small intestine is difficult to visualize, endoscopy may not be able to detect small bowel carcinoids.
• Computed tomography (CT) scans and magnetic resonance imaging (MRI). These imaging tests are used to take pictures of the gastrointestinal tract and pancreas. They can be used to find the primary tumor and to determine whether the tumor has spread.
• Somatostatin receptor scintigraphy (Octreoscan^®). Somatostatin receptor scintigraphy (SRS) is another technique used to identify GEP NE tumors. Many GEP NE tumors contain proteins called somatostatin receptors. These receptors bind the hormone somatostatin, which occurs naturally in the body. Somatostatin receptors can also bind to drugs similar to somatostatin, such as octreotide, which are called somatostatin analogues. To carry out SRS, octreotide is labeled with a radioactive substance called indium-111 and injected into a vein. The radiolabeled octreotide travels through the bloodstream until it meets up with a tumor containing somatostatin receptors and binds to them. A device that measures radiation is then used to detect the radiolabeled octreotide, revealing where the tumor is in the body. The radioactive isotope breaks down and is eliminated from the body in a few days and so is considered safe. This scan can also be used to determine whether somatostatin analogues can be used in treatment for GEP NE tumors, as tumors that carry somatostatin receptors are more likely to respond than those that do not.

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