Also called: Positron Emission Tomography & Cancer
Mark Oren, M.D., FACP
Positron emission tomography is an imaging test used to diagnose and monitor cancer and other conditions. Commonly known as a PET scan, this test helps physicians to detect biochemical changes that may suggest the presence of cancer or other illnesses. The changes on a PET scan may appear before a patient exhibits visible symptoms. For cancer patients undergoing treatment, PET scans can identify how aggressive a cancer is and the extent of its metastasis (spread) to other parts of the body.
PET scans are performed using a scanning device that looks like a large doughnut. Patients are placed on a table and moved into the hole of the scanner. Within the scanner are multiple rings that detect changes in the body and produce images on a computer screen.
As part of the procedure, a form of slightly radioactive sugar is injected into the patient prior to the scan. The cells of the body absorb this sugar, which releases atomic particles called positrons. These positrons combine with electrons in the body to produce gamma rays. As gamma rays leave the body, they are detected by a special camera.
The images from the scanner contain varying colors or levels of brightness that help physicians identify abnormalities, such as the presence of cancer. For example, cancerous tissues use more energy and absorb more sugar than healthy tissues. For this reason, these malignant areas appear brighter than normal tissues on PET scan images. PET scans can also detect differences in both cancerous tissue and normal tissue that may have been damaged by cancer treatment.
PET scans are becoming more commonly used for studying cancer and its spread in the head and neck (including the brain), esophagus, breast, colorectal tract, ovaries and lungs. They may also be used to monitor lymphomas and melanoma. In some cases, PET scans may be used to diagnose or monitor other abnormalities, including those of the heart and brain.
A radiologist or other nuclear medicine expert examines the images from a PET scan and sends a report to the patient’s physician. Results usually are available within a few days of the scan. Depending on the results of this test, additional tests or treatment may be recommended. For patients with cancer, treatment methods may include chemotherapy, radiation therapy, surgery or other therapies.
PET scans are safe procedures that do not pose significant risks for most patients. The radioactive materials used are minimal and only remain in the body from a few hours to a few days. However, pregnant and breastfeeding women may be urged not to undergo a PET scan because of the potential for exposing a fetus or newborn to potentially harmful radiation. For many cancer patients, the benefits of the scan and the information it provides about the disease outweigh the associated risks.
About PET scans & cancer
Positron emission tomography is an imaging technique used to diagnose a variety of medical conditions, especially certain cancers. Commonly known as a PET scan, it is a form of radionuclide imaging in which tiny amounts of radioactive materials (called “tracers”) are injected into a patient’s body. The tracers emit a type of energy called gamma rays, which are detected by special devices and used to generate an image of the body.
Unlike other imaging techniques (e.g., CT scan, MRI), PET scans allow physicians to study changes in how the body functions, which may in turn reveal alterations in biochemical processes. Such changes often suggest the possible presence of illness before visible changes are apparent in a person’s anatomy.
PET scans are performed with a round scanning device with a hole in the center, similar to a doughnut. As part of the procedure, a form of minimally radioactive sugar (fluorodeoxyglucose) is injected into the patient. The cells of the body absorb this sugar, which then release atomic particles called positrons. These positrons combine with electrons in the body to produce gamma rays. When the gamma rays leave the body, they are detected by a special camera called a photomultiplier-scintillator detector.
Multiple rings of detectors inside the PET scanning machine record the emission of gamma rays and create an image that is displayed on a computer monitor. In this way, PET scanning provides pictures of internal structures and information about their activity. Physicians analyze different colors and degrees of brightness on a PET scan to diagnose a patient’s condition.
PET scans are frequently used to reveal the presence of cancer. For example, on a PET scan image, healthy tissue that uses a moderate amount of glucose for energy will appear less bright than cancerous tissue. Malignant areas use greater amounts of glucose and appear as “hot spots” on images. PET scans may be able to detect cancer more accurately than other forms of testing.
PET scans are particularly precise in detecting larger and aggressive tumors, but may be less accurate in detecting cancers that are smaller than 8 millimeters. They may also reveal cancer that has metastasized (spread) from its site of origin to other organs or tissues.
PET scans can reveal biochemical changes in cancerous tissues and may be used to monitor the effectiveness of cancer treatments. The images can reveal whether or not cancer cells are using less glucose, which indicates that they are dying. PET scans are also used to detect cancer that has returned after being successfully treated.
Other conditions also may be revealed through PET scans. These tests may be used to examine the heart and how well it is functioning. PET scans are also performed on the brains of patients who have signs of memory disorders, brain tumors or seizure disorders.
During the positron emission tomography scan
Positron emission tomography (PET) is typically an outpatient procedure. However, the equipment necessary to perform the test usually requires the scan to take place in a hospital’s radiology or nuclear medicine department. In some cases, the PET scanner may be a mobile unit that is brought to a facility for test days.
Patients may be asked to remove articles of clothing and change into a hospital gown for the PET scan. Prior to the PET scan, a machine (called a cyclotron) is used to create a radioactive substance that is attached (tagged) to a natural compound in the patient’s body. Most often, this compound is sugar. However, water or ammonia also may be used.
In many cases, an initial scan will be made before the injection of the radioactive compound. Patients lie down on an examination table with their arms placed over the head. The initial scan takes about 10 to 30 minutes.
The radioactive substance is administered to the patient through an intravenous injection or as an inhaled gas. It generally takes between 30 to 90 minutes for the substance to travel through the body and to settle in the tissue. Patients rest quietly during this time period and avoid excessive movement or talking, which can alter how the substance travels through the body.
During the test patients are asked to lie on a table, which then slides into the hole of the scanner and moves the patient into the machine. Lying completely still during the scan is vital for accurately capturing images. Occasionally, patients may be instructed to shift positions so that images can be taken from different views.
The PET scanner device moves over the table six or seven times during a 45- to 60-minute period. Images are captured by a device called a photomultiplier-scintillator detector and combined with images from a CT scan (computed axial tomography) to create detailed images.
Cancerous tissue or tumors that are present absorb more of the radioactive sugar than healthy tissues and show up brighter in PET scan images. In addition, PET scans can measure cellular and tissue metabolism, show blood flow and determine brain changes after trauma or drug abuse, among other uses.
Although PET scans are not painful except for the brief discomfort of the injection, some patients may become uncomfortable after lying on the table and holding a position for a long period of time. Patients who are claustrophobic may become anxious while positioned in the scanner. Some centers may have open scanners available.
After the positron emission tomography scan
After the test, patients can change back into their clothes if they have been given a gown. There are no restrictions on daily routine, but it is recommended that patients drink plenty of fluids to flush the tracer from the body.
The radioactive materials in the body release their energy and decay into non-radioactive atoms within a few hours to a few days. The day after the scan, patients should immediately flush the toilet after using it to limit exposure to potentially radioactive material. Patients should talk to their physician about whether or not they should limit time around children or refrain from sexual activity for a period of time.
A radiologist or other nuclear medicine expert will examine the images and send a report to the patient’s physician or other members of the cancer care team. Results usually are available within a few days of the scan. Depending on the results of this test, additional tests or treatment may be recommended. The scans may indicate whether a given treatment is succeeding in fighting the cancer and stopping its spread.
Potential risks with PET scans
Positron emission tomography (PET) is generally safe for the patient. Radiation doses are very small, and radionuclides have a low risk of triggering toxic or allergic reactions. Some patients may experience pain or swelling at the injection site. Applying warm, moist cloths usually relieves such symptoms.
Women who are or may be pregnant may want to avoid PET scans because they can expose a fetus to potentially harmful radiation. Because fetuses have fast-dividing cells and are undergoing organ development, they are more vulnerable to damage caused by radiation. Even though radiation levels during these tests are generally considered to be too low to harm the fetus, some physicians recommend that pregnant patients not undergo these tests.
In addition, the tracers used in PET scans can pass from the mother’s breast milk to an infant during breastfeeding. Therefore, a woman should inform her physician if she is pregnant or nursing before undergoing these tests.
Questions for your doctor about PET Scans
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor or healthcare professional the following questions regarding PET scans:
- Why is a PET scan more appropriate for me than other imaging tests?
- Do PET scans pose any potential risks or dangers to me?
- If I am pregnant or nursing, should I delay the PET scan or have another test instead?
- When and where will my test take place?
- Do I need to do anything to prepare? Should I change my diet prior to the test?
- How long will my test take? May I leave the facility during the waiting period?
- When and from whom will I receive my PET scan results?
- What are the next steps after I get my test results?
- What do the test results indicate about my cancer treatment?
- What precautions do I need to take after my PET scan?
- How often will I need to have PET scans?