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. The level of detail they provide makes them an excellent choice for evaluating what should not be there, and how big it is. CT scans were the primary imaging tool for many years and are still one of the primary imaging techniques.
PET scans provide detailed information about metabolic activity. They are not so good at providing information about size, shape and structure. A PET scan shows 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.
Combined PET/CT scanning machines are becoming more common so as you read through this information it will be important to keep that in mind. When a combined PET/CT scan is used, the difference between the two types of scan is a moot point since both scans are being done at the same time. However it is also important to note that a combined scan exposes the patient to significantly more radiation than either scan alone.
Of course another issue of great importance is deciding how often to scan a patient, whether it is by PET or by CT. We have a section on our general imaging page dedicated to this topic.
Click here to go to the section about how often scans should or should not be used
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)
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.
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.
Click here to see what a PET scan looks like (and a combined CT/PET)
The results of the PET scan show up on the image and are quantified using the Standard Uptake Value (SUV). This is a numerical expression of just how much uptake there is at any given place. The higher the number the more uptake there is. When the uptake is in a tumour mass, the higher the number the more rapidly the mass is consuming sugar and growing. Typically a number of 2 or less is normal. A number of 10 or below is considered an indolent lymphoma and a number higher than 10 is likely a more aggressive lymphoma, but there is considerable overlap between the two. Radiologists use the Deauville 5 point scale to interpret the results. (1) (2)
PET scans are best at detecting metabolic activity in the more aggressive types of lymphoma such as diffuse large B-cell lymphoma. It is in these aggressive PET avid types of lymphoma that there is broad consensus that PET should be used.
For indolent lymphomas there is no consensus that PET scans should be used. Some of these types of lymphoma will not show up on PET scans very well or at all. In these types of lymphoma CT is sufficient for imaging. (3)
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 also excellent at determining if a mass left over after treatment is just scar tissue or still active cancer. CT scans cannot distinguish between scar tissue and live cancer tissue but they do see the mass. If the mass is scar tissue it will not been seen by PET but will be seen by the CT scan.
One of the primary strengths of a PET scan is that it is very good at predicting the prognosis for patients. Early studies showed that if the patient had a clear PET scan midway through their treatment then their chances of relapse were significantly reduced. (with respect to DLBC and other aggressive lymphomas) (4) (5) (6) The difficulty was that some patients do not have a clear PET scan midway through, but do have a clear PET scan at the end of treatment. That also proved to be equally accurate at predicting risk of relapse. (7) (8) Therefore there is debate about the usefulness of a midway scan since it might unnecessarily cause the patient anxiety if it is not clear. There is also the problem that unless a positive PET scan midway can change the treatment strategy, what purpose does it serve to do one that early?
Having said all that, even a clear PET scan at the end of treatment for DLBC is not a guarantee that no relapse will occur. Some patients with a clear PET scan at the end of treatment (in the neighbourhood of 14%) will still relapse eventually (9)
People who have transformed indolent lymphoma also generally have a high SUV value that shows up well on PET scan. This can be a useful tool when the patient has a history of indolent lymphoma that has relapsed and is suspected of having transformed. (10)
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. Imagine you have a positive midway 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. It simply exposed you to more radiation. This calls into question the value of doing mid-therapy scans. (11)
The biggest limitation to PET scans is their tendency to have false positives. In one 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. (11) Clearly PET is not perfect and false positives are common. (12) (13) (14) (15) (16)
In order to reduce the chances of a false positive it is important to do the PET scan at the right time. It should be done at least 6-8 weeks after chemo-immunotherapy, and 8-12 weeks after radiation therapy. (17)
PET/MRI for the evaluation of patients with lymphoma: initial observations