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Q & A on PET/CT technology with Dr. James Fletcher[return to Sound Medicine home] >> What is PET/CT? PET/CT is an exciting new diagnostic imaging procedure that combines the best of two different radiology procedures to give doctors a new way of looking at disease. CT or CAT (Computerized Axial Tomography) scanning provides detailed structure and anatomic information but has the intrinsic weakness of almost too much data. It is like a detailed architectural plan. It tells nothing about function or metabolism. A live organ looks pretty much like a dead organ. The PET (positron emission tomography) scanning provides unique information about organ function and metabolism but has the intrinsic weakness of very little anatomic detail -- sort of like an impressionist painting. >> How do CT and PET each make images of the body? Each use a different method for making images of the body and of disease. CT is very similar to a standard x-ray in that it uses an x-ray source to make its pictures. PET uses special compounds called radiopharmaceuticals which are handled in a very specific way by the body. The most common radiopharmaceutical for PET today is a labeled sugar called fluorodeoxyglucose or FDG. >> How does the FDG work? This is a labeled sugar. It is labeled with a very small amount of a radioactive molecule that is given to the patient which then tells doctors how cells in the body use the sugar. It is quite fortunate for us that tumor cells use the sugar in a way that is different than normal cells. This difference allows us to see tumors in the body with an accuracy which is much greater than CT. The tumor cells literally light-up and provide us with a new way of identifying the location of cancer cells or tumors. >> Why do you need CT if PET is so much better? The localization of FDG by some tissues can be normal and FDG uptake can also be seen in some non-cancerous tissues. CT provide important anatomic detail that PET does not have to help doctors understand the precise location of the FDG. This often helps sort out if the FDG uptake is in normal tissues rather than cancerous tissues. >> What types of cancers can be evaluated by PET/CT? It is probably easier to answer by indicating which types of common tumors are not seen by PET/FDG: prostate, liver, pancreas and kidney. PET/CT with FDG works very well for common cancers of the head and neck, thyroid, lung, esophagus and lymphoma, myeloma, colorectal cancer, melanoma and certain brain tumors. New PET radiopharmaceuticals are under development that will be able to help doctors diagnose some of the other cancers where FDG does not work well. At IUSM we will be studying a new PET compound called fluorocholine which promises to work very well in cancer of the prostate. >> How does PET/CT help patients with cancer? In most instances, the patient already has a diagnosed cancer. Before a decision can be made about how to treat the cancer, the doctor must know the stage of the cancer or how far it has spread throughout the body. Treatment will differ depending on the stage of the cancer. PET/CT is often used to determine the stage of the cancer before any initial therapy is provided. PET/CT is also used to evaluate for recurrence of cancer after it has been treated. CT alone often is unable to make this determination. PET/CT is used to determine if a certain therapy is working or if the cancer is resistant to the treatment. New studies are in progress to see if PET/CT can identify these resistant cancers early in treatment so that the patient can receive alternate therapies that do work. PET/CT is used to evaluate "indeterminate radiographic masses." This is translated to mean doctors cannot tell if a lump or bump is malignant with obtaining a piece of the tissue by biopsy or surgery. If the lump or bump is not seen on PET it is very likely not malignant. This often can spare the patient an unnecessary biopsy or surgery. These patients can then be followed carefully with repeat CT and PET/CT scans. IF the PET is positive, then it is necessary to proceed with biopsy or surgery as the lump or bump has a good chance of being malignant. PET/CT is also beginning to play an important role in treatment planning using radiation therapy. >> Are there other diseases besides cancer that can be studied by PET/CT? Yes, in cardiac disease and in dementia and epilepsy. PET/CT can help doctors determine if certain patients with advanced coronary artery disease and heart failure will benefit from bypass surgery or if they might need a heart transplant. PET/CT can help doctors establish the diagnosis of dementia. In particular, PET is very accurate in identifying Alzheimer's disease in its very early stages. No other test can provide the accuracy of PET except for brain biopsy. >> What is the future of PET/CT? PET provides a way of understanding disease at a molecular or chemical
level. Understanding the genome has begun to give us a road may to understanding
the molecular basis of disease.
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