Pet scans


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PET scans and lymphoma

This page is here because there is a lot of confusion about PET scans and their role in lymphoma. It is a common misconception that PET scans are "better" than CT scans or other types of diagnostic imaging. This simply is not true. PET scans are "different" and provide quite different information but they are not better.

CT scans provide a picture of the size, shape and structure of what is inside the body. Because of this level of detail that they provide they are the primary type of scans used for lymphoma.PET scanner

PET scans provide detailed information about metabolic activity. They are not so good at providing information about size, shape and structure. All they show is what tissues are metabolically active, and the exact location. But like many types of medical tests, they are also subject to false positive or even false negative results.

This page is divided into three sections. The first section discusses what PET scans are, and shows some of the images they produce. The second section deals with what PET scans are good for, and how they are being used. The final section discusses what PET scans are not good at, and their limitations.

Perhaps the best place to start however is the USA National Comprehensive Cancer Network recommendations on the use of PET in cancer.

NCCN Task Force Report: PET/CT Imaging in Cancer (pg 18 for lymphoma)


What is a PET scan

PET scans detect metabolic activity. That means they can see cells that are consuming sugar for energy. PET scans are very good at detecting metabolic activity where it does not belong. The patient is injected with a sugar molecule that has radioactive Fluorine attached. It is 18-Fluorodeoxyglucose or 18-FDG.

In most cases after the injection the patient must lie or sit quietly for about an hour to allow the 18-FDG to distribute about the body. They must remain calm because movement can cause that part of the body to require more energy and absorb more sugar. Even chewing gum will cause a problem.

Next the patient goes into the scanner and the scanner detects where the radiation is accumulated in the body. It does this because the radiation is gamma radiation that is exiting the body and can be detected by the gamma camera (aka PET scanner)

Parts of the body that require a lot of energy will absorb the 18-FDG and therefore show up on the image. There are places that SHOULD show up, like the brain, heart, bladder, kidneys etc. These places are all very active and need energy.  What the radiologist will look for is "abnormal" accumulation in places where it does not belong.

Those who have low-grade or indolent forms of lymphoma will benefit less from the use of PET. The slow growing nature of these types of NHL means that the tumours are not growing quickly and thus are less likely to attract the glucose. While most types of indolent lymphoma will show up to some extent on a PET scan there are some types which will fail entirely to show up. The important point though is that indolent lymphomas are generally only treated when the tumour burden begins to impact the quality of life, or threaten major organs. These conditions are easily detected by the patient (quality of life) or the CT scan (threatening major organs) so the cost of a PET scan can't usually be justified.

Furthermore the PET scan is unlikely to change the treatment course so its value in low-grade lymphomas is limited.   A more recent advance is the combination CT and PET scanner. This machine does both types of scan at the same time, which is far more cost effective. It also allows the radiologist to combine the images for a more detailed view of what is going on. Click the first link above to see combined CT/PET images. The following links will help you understand when and why to use PET scans.

What can PET scans do well

PET scans are good at finding metabolic activity where it does not belong. Lymphoma (and other cancers) often have high metabolic activity since they are growing more rapidly than they should. This means PET scans are good at finding small areas of cancer that don't show up on CT scans. PET is excellent at determining if a mass left over after treatment is just scar tissue or still active cancer.

PET scans done in the middle of treatment for aggressive lymphomas are thought to be good at providing prognostic information. In other words if your PET scan midway through treatment is negative you have an excellent prognosis for not relapsing. But more recent information is brining this into dispute. Not only do some studies show that those who have a positive PET scan still have a good prognosis, but there is increasing evidence showing that there is little consensus about whether a PET is positive, or negative. Some experts read it one way, and others read it the other way. There is no standardized method for interpreting the results of a PET scan. Here is a study regarding this problem, and another authors comments on the study.

Here is another study that sheds some light on the value of mid-treatment PET/CT scanning. In summary they find that they are not necessary because a scan after treatment is over is just as good at predicting outcome. Therefore the extra radiation of a mid-treatment plan may not be a wise idea.

Another challenge is that when a mid treatment PET scan is positive it is still not clear how to use this information to choose better treatments. Until that is possible PET scans are more informational than they are decisive.

What are the limitations on PET scans

Like any type of diagnostic test, PET scans have limitations such as false positive results. But perhaps the biggest limitation is that although PET scans provide excellent prognostic information they don't always provide any information that will help or change the treatment recommendations. A test is only as good as the benefits it can provide. If you have a bad PET scan and you ask the doctor, "How should we change my treatment plan based on this scan?" If the doctor replies, "The scan does not change the treatment strategy." then what was the point of the scan.


Below are some studies which examine these limitations.  The first and second studies below have quite surprising results. In the first study after 4 cycles of R-CHOP patients with DLBC had a mid-treatment PET scan. All patients with a positive PET scan were required to have a repeat biopsy to confirm the finding. Surprisingly of the 38 PET positive patients 33 of them turned out to be negative by biopsy. Clearly PET is not perfect and false positives are common.

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