PET and CT - FAQs
Positron Emission Tomography (PET) is rapidly becoming a major diagnostic imaging modality used predominantly in determining the presence and severity of cancers, neurological conditions, and cardiovascular disease. It is currently the most effective way to check for cancer recurrences. Studies demonstrate that PET offers significant advantages over other forms of imaging such as CT or MRI scans in diagnosing disease. Last year more than 200,000 PET scans were performed at more than 700 sites around the country. If you're interested in learning how a PET scan can benefit you and need additional information, talk with your local health care provider or referring physician. At the end of this page are links to other sites with PET information too.
PET images demonstrate the chemistry of organs and other tissues such as tumors. A radiopharmaceutical, such as FDG (fluorodeoxyglucose), which includes both sugar (glucose) and a radionuclide (a radioactive element) that gives off signals, is injected into the patient and its emissions are measured by a PET scanner.
A PET scanner consists of an array of detectors that surround the patient. Using the gamma ray signals given off by the injected radionuclide, PET measures the amount of metabolic activity at a site in the body and a computer reassembles the signals into images. Cancer cells have higher metabolic rates than normal cells, and show up as denser areas on a PET scan. PET is useful in diagnosing certain cardiovascular and neurological diseases because it highlights areas with increased, diminished or no metabolic activity, thereby pinpointing problems.
How do you use PET during cancer diagnosis?
Normal body cells grow, divide, and die in an orderly fashion replacing worn-out or dying cells and repairing injuries. Sometimes these normal cells begin to grow uncontrollably. These abnormal cells outlive the bodies normal cells and continue to grow and divide forming new abnormal cells. The mass of extra cells forms a growth or tumor, which can be benign or malignant. They are not cancerous.
Benign tumors can be removed low probability of recurrence and are usually not life threatening. But if the tumor is malignant it is cancerous. In a process called metastasis, cells from a malignant tumor can break off and travel to other parts of the body via the bloodstream or lymphatic system where they begin to grow and replace normal tissue. Cells from a malignant breast tumor can spread to another organ, for example the brain. Although these cells are in the brain, the cancer is still breast cancer.
PET is considered particularly effective in identifying whether cancer is present or not, if it has spread, if it is responding to treatment, and if a person is cancer free after treatment. Cancers for which PET is considered particularly effective include lung, head and neck, colorectal, esophageal, lymphoma, melanoma, breast, thyroid, cervical, pancreatic, and brain as well as other less-frequently-occurring cancers.
How Can PET/CT Make a Difference?
Early Detection: Because PET images biochemical activity, it can accurately characterize a tumor as benign or malignant, thereby avoiding surgical biopsy when the PET scan is negative. Conversely, because a PET scan images the entire body, confirmation of distant metastasis can alter treatment plans in certain cases from surgical intervention to chemotherapy.
Staging of Cancer: PET is extremely sensitive in determining the full extent of disease, especially in lymphoma, malignant melanoma, breast, lung, colon and cervical cancers. Confirmation of metastatic disease allows the physician and patient to more accurately decide how to proceed with the patient's management. PET shows whether or not a tumor is benign or malignant. No other imaging technique can do this! Reports in the scientific literature find that PET correctly identifies detected lesions 97% of the time. Painful, invasive surgery, such as thoracotomy, may no longer be necessary for diagnosis. PET shows the extent of disease -- called staging -- of lung cancer, colorectal cancer, melanoma, head and neck cancer, breast cancer, lymphoma and many other cancers. For patients whose cancer is newly diagnosed, it is important to determine if the cancer has spread to other parts of the body, so that appropriate treatment can be started. PET can search the entire body for cancer in a single examination, called a "whole body scan", revealing any metastases as well as the primary site.
Checking for recurrences: PET is currently considered to be the most accurate diagnostic procedure to differentiate tumor recurrences from radiation necrosis or post-surgical changes. Such an approach allows for the development of a more rational treatment plan for the patient.
Assessing the Effectiveness of Chemotherapy: The level of tumor metabolism is compared on PET scans taken before and after a chemotherapy cycle. A successful response seen on a PET scan frequently precedes alterations in anatomy and would therefore be an earlier indicator of tumor response than that seen with other diagnostic modalities.
How does PET compare to MRI and Computed Tomography (CT)
Because PET measures metabolism, as opposed to MRI or CT, which "see" structure, it can be superior to these modalities, particularly in separating tumor from benign lesions, and in differentiating malignant from non-malignant masses such as scar tissue formed from treatments like radiation therapy. PET is often used in conjunction with an MRI or CT scan through "fusion" to give a full three-dimensional view of an organ and the location of cancer within that organ. Newer PET scanners are being made that are a combination of PET/CT devices.