The Role of PET Scans in the Diagnosis and Treatment of Cancer
by Dr. C.H. Weaver M.D. updated 2/2019
What is a PET scan?
Positron emission tomography is an imaging test used to show how your organs and tissues are working. Whereas other imaging tests, such as X-ray, CT, and MRI, reveal structural changes in the body, PET is used to reveal chemical and physiological changes. Uses for PET scan include checking brain function; diagnosing cancer, heart problems, and brain disorders; examining blood flow to the heart; and determining spread of cancer and response to therapy.
The use of PET scans may help doctors more accurately detect the presence and location of cancer cells. A PET scan is similar to a CT scan; however, PET scans can detect live cancer tissue.
Prior to a PET scan, the patient receives an injection of a substance that contains a type of sugar attached to a radioactive isotope. The cancer cells “take up” the sugar and attached isotope, which emits positively charged, low energy radiation (positrons). The positrons react with electrons in the cancer cells, which create the production of gamma rays. The gamma rays are then detected by the PET machine, which transforms the information into a picture. If no gamma rays are detected in the scanned area, it is unlikely that the mass in question contains living cancer cells.
The PET scan can measure blood flow, oxygen use, how your body uses sugar, and much more.
How is a PET scan performed?
Before a PET scan the radioactive tracers are given through a vein in your arm, through a solution you drink, or in a gas you inhale. Because your body needs time to absorb the tracers, you’ll wait about an hour before the PET scan begins.
The actual scan can last anywhere from 30 to 45 minutes. This will involve lying on a narrow table attached to a PET machine, which looks like a giant letter “O.” The table slides slowly into the machine so that the scan can be conducted.
During the scan you need to lie still. You may even be asked to hold your breath for several seconds. You’ll hear buzzing and clicking noises during the test. When all the necessary images have been recorded, the table will slide back out of the machine and PET scan is complete.
Why is a PET scan performed?
A PET scan is used to inspect blood flow, oxygen intake, or the metabolism of organs and tissues. PET scans show problems at the cellular level, giving your doctor the best view of complex systemic diseases.
PET scans are most commonly used to detect:
- Cancer-Cancer cells have a higher metabolic rate than non-cancerous cells. Because of this high level of chemical activity, cancer cells show up as bright spots on PET scans. For this reason, PET scans are useful for detecting new or recurrent cancers.
- Heart problems-PET scans reveal areas of decreased blood flow in the heart. This is because healthy heart tissue will take in more of the tracer than unhealthy tissue or tissue that has decreased blood flow. Different colors and degrees of brightness on the scan will indicate different levels of tissue function, helping you and your doctor decide how best to move forward
- Brain disorders-The sugar glucose is the main fuel of the brain. During PET scans, tracers are “attached” to compounds such as glucose. By detecting radioactive glucose, the PET scan is able to detect which areas of the brain are utilizing glucose at the highest rates. PET scans are used to help diagnose and manage many central nervous system (CNS) disorders.
How does the PET scan compare to a CT or MRI scan?
PET scans show metabolic changes occurring at the cellular level in an organ or tissue. This is important because disease often begins at the cellular level. PET scans can detect very early changes in your cells. CT scans and MRIs uses special x-ray equipment or magnetic fields and radio frequency pulses to create images of internal structures such as organs, soft tissues, and bone respectively.
Sometimes a PET–CT or a PET–MRI scan are combined. When either of these scans is performed in conjunction with a PET scan, they result in what’s called image fusion. A computer combines the images from the two scans to create a three-dimensional image, which provides more information and allows for a more precise diagnosis.
Are there risks involved with getting a PET scan?
The PET scan involves the use of radioactive tracers, but the exposure to harmful radiation is minimal. The radioactive tracer is typically attached to the sugar glucose which makes it easy for your body to eliminate the tracers. It’s possible to have an allergic reaction to the tracer. People who are allergic to aspartame, saccharin, or iodine should alert
What happens after a PET scan?
After the test, you can go about your day unless your doctor gives you other instructions.
However, because radioactive material will remain in your body for about 12 hours, you’ll want to limit your contact with both pregnant women and infants during this time.
Drink plenty of fluids after the test to help flush the tracers out of your system. Generally, all tracers leave your body after two days.
Will I be exposed to radiation if I have a PET scan?
You will be exposed to radioactive material when you receive a PET scan, but—unless you are pregnant—the amount of radiation is considered too low to be harmful. As well, the radioactive material does not stay in your body very long.
Can I receive a PET scan if I’m pregnant or breast feeding?
Although PET scans are considered safe for the general population, pregnant women and women who are breast feeding need to know that the radioactive material may harm the fetus or infant. Women and their doctors must carefully weigh the necessity and benefits and risks of having a PET scan while pregnant or breastfeeding.
Is it possible to have a bad reaction to a PET scan?
Rarely, patients have an allergic reaction to the radioactive substance. Reactions may occur where the radioactive substance is injected and involve pain, redness, or swelling. Again, this type of reaction is rare.
PET Scans Have Improved the Management of Several Types of Cancer
PET Scans May Improve Treatment for Esophageal Cancer
Clinical trial results presented at the 2017 Gastrointestinal Cancers Symposium in San Francisco suggest that the use of PET scans to assess a cancers response to initial chemotherapy may provide doctors with useful information to further tailor chemotherapy.
Standard treatment of patients with stage II-III esophageal and gastroesophageal junction (GEJ) cancers includes chemotherapy with radiation (chemoradiation), followed by surgery. Physicians may use several different chemotherapy regimens without knowing which particular chemotherapy will be most effective.
In the current study doctors administered chemotherapy before chemoradiation and used PET scans after the initial chemotherapy to assess response to treatment in order to see if the early PET scan can help doctors make quick course corrections to maximize patient benefit from chemotherapy.
A total of 257 patients with stage II-III esophageal and GEJ adenocarcinoma were treated with one of two different chemotherapy regimens: either modified FOLFOX-6 or carboplatin/paclitaxel. A PET scan was repeated in all patients after a couple cycles of chemotherapy. If the PET scan demonstrated that the chemotherapy was working, no change in treatment was made. If the PET scan however demonstrated that the chemotherapy was not effective, the chemotherapy was changed to the other regimen.
Overall, 39 out of 129 patients who started treatment with the modified FOLFOX chemotherapy, and 49 out of 128 patients who started with carboplatin/paclitaxel, switched chemotherapy regimens after the results of the PET scan demonstrated treatment was ineffective.
For patients who switched to the alternative chemotherapy almost 16% achieved a complete disappearance of cancer. This is significantly higher than historical experience. Although improvement in chemotherapy regimens and additional studies are ultimately needed this study suggests that PET scans may be a valuable tool to help oncologists individualize treatment for their patients.
Thyroid Cancer Medicare Now Covers PET Scans for Diagnosis of Thyroid Cancer
The Department of Medicare and Medicaid recently announced the approved coverage of PET scans for the diagnosis of thyroid cancer.
The department of Medicare and Medicaid have determined the effectiveness of PET scans in diagnosing thyroid cancer and are now offering coverage for this indication.
PET Scan Superior to Conventional Imaging for Breast Cancer
According to an article published in The Journal of Nuclear Medicine, whole-body positron emission tomography (PET) appears more accurate than conventional imaging (CI) in predicting the risk of recurrences in breast cancer patients.
Imaging procedures that are often utilized prior to and following treatment to determine where the cancer may exist in the body include computerized tomography (CT) scans, x-rays, sonography, magnetic resonance imaging (MRI), mammography and bone scans. More recently, PET scans have been utilized and appear to have superior imaging to more conventional scans for various cancers.
Researchers from Sacramento evaluated the accuracy of PET scans in determining the risk of recurrences in patients who had been recently treated for breast cancer. In this study, 61 patients were evaluated at an average of .4 years following their last treatment. The patients were first evaluated with CT scans, X-ray, MRI, mammography, bone scans and sonography and then re-evaluated with one whole-body PET scan. The PET scan accurately determined the outcome of 90% patients, while the combination of all the conventional images accurately determined the outcome of only 75% of patients. Of the patients in which there was a discrepancy between the PET scan and the other imagings, PET correctly predicted the outcome of 80% of patients while the conventional imagings correctly predicted the outcome of only 20%.
These results appear to indicate that one whole-body PET scan may more accurately predict the risk of a recurrence than multiple conventional imagings in patients recently treated for breast cancer. This may allow patients to receive more appropriate follow-up treatment in order to provide optimal chances for long-term survival or a cure. Medicaid and Medicare have recently approved reimbursement for PET scans in women with breast cancer. Reimbursement includes staging for distant metastases, restaging patients with local recurrences or metastasis and determining treatment response. Patients with breast cancer may wish to speak with their physician about utilization of a whole-body PET scan.
PET Scans Can Help Predict Outcomes in Non-Hodgkin’s Lymphoma
Results from a positron emission tomography (PET) scan can help predict outcomes for patients diagnosed with non-Hodgkin’s lymphoma. Patients with a predicted poorer outcome may wish to undergo more aggressive therapy than those predicted to have optimal outcomes.
A current area of research involves efforts to individualize treatment in order to provide optimal outcomes for all patients. Although standard therapeutic approaches exist for different types of NHL, it is not clear why some patients respond to certain treatments while others may not. A process where scans follow initial cycles of treatment to test anti-cancers responses to treatment is currently being evaluated. Patients who appear to be responding well to treatment may continue with the treatment, while others who are not responding as well may alter their treatment strategy or try a more aggressive approach. Researchers are trying to appropriately match different types of scans with various cancers to provide information that may alter the course of care.
The use of PET scans may help doctors more accurately detect the presence and location of cancer cells. Researchers from London conducted a clinical trial to evaluate the effectiveness of a PET scan in NHL. This trial included 121 patients who underwent a PET scan following two to three cycles of chemotherapy. The response to therapy according to PET results corresponded with a patient’s progression-free survival and overall survival at 5 years. Of the patients whose results from PET scans were considered negative (good responses to therapy), a majority (89%) were alive without disease progression at 5 years. Conversely, among those whose PET scan results were considered positive (poor response to therapy), progression-free survival was only 16% at 5 years. The associations between PET results and PFS at 5 years were made independently of other factors that typically indicate prognosis (such as stage of disease).
The researchers concluded that PET scans following two to three cycles of chemotherapy may help to guide treatment options in aggressive NHL. Those with a poorer prognosis according to their PET scan results may wish to undergo alternative therapeutic approaches for optimal outcomes, while those with a good prognosis according to their PET scan may wish to continue with planned therapy. Although further studies are required regarding the clinical utility of PET scans in aggressive NHL, patients with this disease may wish to speak with their physician regarding their individual risks and benefits of undergoing a PET scan, as well as altering their therapeutic strategy should their results indicate a poor prognosis.
PET Scans Improve Accuracy of Staging of Patients with Melanoma and May Lead to More Appropriate Treatment
Accurate staging (extent of cancer) of melanoma is important to determine the most effective treatment for the patient. The use of PET scans can improve accuracy in the detection of the presence and location of melanoma, which may ultimately lead to better treatment.
Researchers at Duke University evaluated the effectiveness of PET scans to detect melanoma in 95 patients. Clinical tests, including CAT scans, had determined that all of these patients had stage III melanoma, with no evidence that the cancer had spread. Following the staging, the patients all had a full body PET scan. Twenty percent of the areas detected by the PET scan were previously undetected cancer cells that had spread. These findings lead to a change in the planned treatment for 15% of these patients.
These findings indicate that PET scanning can be helpful in more accurately evaluating the extent of melanoma, which ultimately dictates the treatment strategy. The physicians conducting this study currently utilize PET scanning as initial staging for patients who have stage III melanoma. Any abnormality seen on PET is then evaluated further with a localized CAT scan to better define the abnormality. It seems reasonable that this approach should help maximize the detection of unsuspected cancer in staging patients with melanoma. Importantly, positron emission tomography scans now have a Medicare reimbursement for melanoma patients.
PET Scan Can Help Identify Treatment Options for Men With Prostate Cancer and Rising PSA
A recent study suggests that PET scans can identify which prostate cancer patients would benefit from additional radiation treatment.
Many individuals treated for prostate cancer eventually develop a recurrence, which are often detected when their PSA levels start rising on routine blood tests. Unfortunately, standard imaging tests aren’t good at determining where cancer cells are located. 68Ga-PSMA PET/CT imaging, or PSMA-PET imaging for short, represents a major advance in detecting prostate cancer.
The PSMA-PET imaging test works by marking an antigen receptor that sits on the surface of every prostate cancer cell, called PSMA, with a radioactive peptide, Gallium-68. This process allows the cancer cells to be detected wherever they are located throughout the body and better detects prostate cancer recurrences.
PET/CT Scans Effective In Locating Pelvic Recurrences of Rectal Cancer
The combined use of PET scans and CT scans is an effective method for detecting pelvic recurrences of rectal cancer.
In this recent trial, 62 patients who had previously undergone abdominal surgery as part of their treatment were referred for PET/CT scanning for evaluation of pelvic recurrences. In total, 81 pelvic sites were found, of which 44 were malignant. Although displaced pelvic organs as the result of surgery did produce some false-positive results, researchers found that PET/CT scans were in the range of 90% specific and sensitive for locating malignant pelvic recurrences.
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