What is the Meaning of FDG Uptake on PET Scan?

FDG is a radioactive sugar that is used in PET imaging.  The idea is that FDG is taken up more in abnormal tissues than normal ones.  That is, abnormal tissues use glucose or sugar more than normal ones.  This is reflected in PET scans as more uptake and a hotter or brighter tissue on the scan.

Is FDG uptake always abnormal?

FDG uptake on PET scan can be normal or abnormal.  For example, the brain uses a lot of glucose and has more FDG uptake then say the lung.  The spleen and liver also have a lot of FDG uptake which is normal.  Fat has much less FDG uptake than the liver and spleen.

FDG uptake can mean cancer in some cases

The usefulness of FDG uptake on PET scans comes in when we want to detect disease, especially cancer.  Cancerous tissues on PET scan will often have more uptake than the background tissue.    This means that a liver tumor will have more FDG uptake than the normal liver.  This will create a bright spot in the liver which becomes suspicious for cancer.

Tissue which is inflamed, infected, bruised or injured in some way can also have high FDG uptake.  Tissue which is infected or inflamed will also have higher FDG uptake than the normal surrounding tissues.  An infection in the liver will also have higher FDG uptake than the surrounding normal liver tissue.  Tissue which is bruised or has undergone surgery may also show high FDG uptake.

What is the diagnosis when we have FDG uptake on PET scan?

FDG uptake on PET scan therefore does not necessarily imply cancer or a specific diagnosis.  FDG uptake has to be correlated with the imaging appearance on CT and the clinical history.   FDG uptake on PET scan can be present for a number of reasons.

For example, a spot in the lung which has FDG uptake in a patient with cancer may represent a metastasis, infection or a benign abnormality.   The radiologist interpreting the scan will use all information to arrive at the correct diagnosis.  He will use any prior scans, the imaging appearance, how much FDG uptake there is and the patients clinical symptoms.  There may be a need for further testing and imaging as well.

FDG uptake can also be used to monitor cancer

FDG uptake on PET is also useful to see if there is a response to treatment.  For example, treated cancer will have a lower FDG uptake if there is a response.  More FDG uptake may mean the tumor is getting worse.  FDG uptake in a new area becomes suspicious for new metastasis or spread of cancer.

FDG uptake can therefore occur in normal and abnormal tissues.  A knowledge of normal distribution of the FDG uptake and the underlying anatomy is therefore crucial to know what is abnormal.  It is also important to review all the available clinical information and prior imaging to arrive at the correct diagnosis.

PET scans are powerful tests that measure FDG uptake throughout the body.  They can identify a tumor.  Stage cancer by seeing if it has spread to other parts of the body.  We can monitor cancer and see if it is responding to treatment.  We can use PET scans to monitor for cancer recurrence.