<b>PET Scans [Positron Emission Tomography ]</b>
are useful for identifying metastases across most of the body, as well as for differentiating between local recurrence of high grade tumors and scar tissue, and can be used in certain situations to examine the impact of chemotherapy on an active tumor. 

PET scans will highlight a high grade, active metabolizing tumor, that is 7mm or larger.  Low grade tumors will not pick up enough radioisotope to distinguish themselves against background, nor will very small tumors.  Sites of inflammation or infection will also pick up extra radioactively tagged glucose, and will highlight areas which may not be cancerous.  It is always necessary to correlate a PET scan with another imaging method, like CT or MRI or ultrasound, to determine what the PET scan has actually found.  

PET scans are the result of an intravenous injection of a solution of radioisotope-tagged glucose molecules, waiting for the molecules to be picked up by highly active cells in the body, and then taking a picture of the resulting "hot spots".  The completed scans look similar to Bone Isotope scans, with dark "hotspots" superimposed upon a small skeleton, with resolution on about the same level.

PET scans are measurements of the metabolic activity of lesions, and screen for metastases that are asymptomatic and still small.  They also can help differentiate a high grade tumor from other lesions that are not cancerous.  They can also be used to discover quickly if there is a response to a chemotherapy agent, e.g. during trials of Gleevec on GIST sarcomas, tumor necrosis could be seen on a PET scan within 8 to 10 days after treatment was begun - but the same tumors didn't shrink enough to be measurable on CT scans until 3 to 4 months later 

"My onc told me about the first time he used PET to examine the before-and-after impact of Gleevec on a GIST patient - 'it was like an aerial view of a city at night, and all the lights had gone out - it was the most exciting thing I have ever seen. The tumors were still there but we could see they had stopped working.' "

PET is expensive (as much as $5000), has frequent false positives, and even false negatives. The parts of the body that can be scanned vary with the equipment available. It should be treated as a screening test.

IF something shows up, then you'd have to do an MRI of the area to actually determine if it was something to be concerned with.

A new combination of PET and CT scan, done at one sitting, will allow for correlation between metabolic activity [PET scan] and the more exact resolution of the lesion and its location  [CT scan].  This combination scan is available at a few centers only at this time. The method combines computed tomography (CT) and positron emission tomography (PET) to create computer-generated images of cancerous tumors.  A CT scan shows precise anatomical detail.  A PET scan is sensitive enough to reveal even very small, high-grade tumors, but its fuzzy image is poorly correlated with a tumor's exact size and precise anatomic location.  The combination of the two scans pinpoints even small deposits of cancer cells and gives the exact location.

Exploitation of PET technology is also involved in:

a. Techniques for adding radioactive ions to chemo are now allowing injection of drugs directly into tumors, while the doctors watch on the PET scanner to see whether the drugs are having an effect. This is being done clinically in trials in the UK.

b. New materials that carry radioactivity and can bind to cells with specific DNA characteristics are also being developed. There is a clinical trial due to start in the UK with high risk breast cancer patients to see whether metastases can be identified before they become tumors.

Exploiting PET technology involves chemists, physicists, and pharmacologists, as well as the electronics wizards, radiologists and other doctors. New skills are being learned, new materials are being created, new treatments are being developed, and new hope too.

NIH/Medline Site
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Information on Imaging of the body
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