April 28, 2005
Current Status of Positron Imaging for Breast CancerTopics: Medical Science News
Positron emission tomography, PET,is currently accepted to be the most accurate way to stage and monitor many types of cancer. PET is a Nuclear Medicine imaging technique that uses very short-lived radioactive compounds that localize in cancer cells. Recent advances in molecular biology research have identified many abnormalities of cellular function in tumors. Cancer cells have alterations in the normal metabolism of the sugar, glucose, and have increased glucose uptake and decreased glucose clearance. Glucose can be bound to the positron 18-F to make a compound 18 F-fluorodeoxyglucose, FDG. When injected into a patient's blood stream, FDG is taken up by the cancer cells at a more rapid rate than normal cells and allows cancers to be seen as "hot spots" on the PET scan. A typical PET scan usually images the body from the head to the upper thighs. Because the brain uses glucose for metabolism, high background activity can mask small lesions, and so, PET has limited value for diagnosing brain metastases.
Blood sugar level affects tumor uptake on PET scans. With high blood glucose, tumor uptake of FDG may be decreased and so patients are asked to fast before the scan. Also, since muscles also use glucose for metabolism, patients are required to lie still for about 60-90 minutes after FDG injection. It is also necessary to curtail any vigorous exercise, just as jogging or weight lifting, for at least 48 hours prior to a PET scan as the recovering muscles will continue to take up glucose for about 24 hours.
Scanners are now available that can perform both PET and CT scans in the same imaging session. The images that are obtained can be overlaid or fused so the functional information of the PET scan can be accurately localized on the high-resolution anatomic images of the CT scan. Fusion imaging performed on these hybrid scanners has been shown to improve whole body scan diagnostic accuracy.
BREAST CANCER DIAGNOSIS:
Although positron imaging is very useful in identifying recurrent of metastatic disease, Medicare coverage for diagnosis of primary breast tumors is currently not approved by HCFA. Tumor size and cell type are factors that effect PET scan accuracy. Although accuracy in detecting tumors larger than 2cm is high, PET may miss approximately one third of invasive cancers smaller that 1 centimeter. PET is more likely to identify invasive ductal carcinoma, to miss invasive lobular carcinomas, and is not helpful for identification of non-invasive tumors. Although the basic mechanism of uptake is via glucose metabolism, other factors that result in this variation in FDG uptake in breast tumors have not been clearly identified. When positive, however, the intensity of PET uptake in a primary breast tumor has been shown to correlate with the degree of tumor aggressiveness.
Care must be taken in interpreting PET scans of patients after biopsy of surgery, as sites of inflammation of infection will also display increased uptake of FDG. With these limitations, PET is not useful as a screening tool. It may, however, provide some benefit when used for problem solving. For example, the value of mammography is often limited after breast augmentation, and in this situation, PET might offer some advantage in screening for cancer. Combining PET and dynamic MRI information in assessment of breast lesions found on mammography may decrease the number of biopsies of benign lesions, as the high specificity of PET complements the high sensitivity of dynamic breast MRI. Currently, a few research centers are evaluating dedicated positron emission mammography devices that may potentially improve identification of small breast cancers.
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Posted by Richard at April 28, 2005 12:16 PM
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