What Does FDG-Avid Mean on a PET Scan?

FDG-avid means an area took up extra tracer during a PET scan. The tracer is called FDG, short for fluorodeoxyglucose, a radioactive form of sugar. PET scanners detect where the tracer collects in the body. Areas that use more sugar than usual often show up brighter on the scan.

Cancer cells often use more sugar than normal cells, to fuel their fast growth. The National Cancer Institute notes that abnormal cells, including cancer cells, can take up more of this tracer than normal tissue. That is why FDG-PET scans can help find cancer, check whether it has spread, or see how well treatment is working.

FDG uptake is not unique to cancer. Infection and inflammation can also use extra sugar and light up on a scan. So can healing tissue after surgery or an injury, active muscles, brown fat, which some people have more of than others, and certain normal organs, such as the brain and bladder. Recent surgery, radiation treatment, injections, or an active infection can all affect how a scan looks and how it should be read.

Your report may include a number called the SUV, or standardized uptake value. This gives a rough measure of how much tracer an area absorbed. It is read alongside everything else, not as a stand-alone verdict. Very high SUVs raise more concern than mildly elevated ones, but there is overlap between what benign and cancerous areas can show.

Your report may describe the location, size, and intensity of uptake, along with whether it is new or has changed since a prior scan. It often compares the PET findings with the CT portion of the same scan, since PET and CT are usually done together. A finding that lights up on PET and also shows a matching abnormality on CT is generally taken more seriously than uptake alone.

FDG-avid findings are sorted into next steps based on how strongly and where they show up. Some are judged low risk and simply monitored. Others prompt more imaging, a biopsy, or a change in treatment plan if they represent cancer that has grown or returned.

FDG-avid results are read as part of a full picture, not treated as an emergency on their own. Still, if you are already in cancer treatment, a significantly new or growing area of uptake usually prompts your care team to move fairly quickly with follow-up.

• Is the uptake expected, indeterminate, or suspicious for cancer?
• What benign explanations were considered for this finding?
• Was a prior scan compared, and has anything changed?
• Is the plan observation, more imaging, biopsy, or a change in treatment?

How you prepare for a PET scan can change what shows up as FDG-avid. Patients are usually asked to fast for several hours beforehand. They are also asked to avoid hard exercise the day before. Active muscles and high blood sugar can both throw off how the tracer spreads through the body. Cold exposure can activate brown fat. This kind of fat then shows up as uptake that has nothing to do with cancer. Follow any instructions your scan technologist gives you closely. This helps make sure any FDG-avid finding reflects something real, not a preparation problem that needs a repeat scan.

Sometimes an FDG-avid finding is unclear. In that case, the best next step may not be a biopsy right away. A repeat PET scan after an interval set by your team can help instead. Infection and inflammation tend to settle down over that time. Cancer typically does not. A repeat scan can help sort out which explanation fits better. This avoids an invasive procedure in many cases. This approach works best for findings judged low to moderate risk at the start. Higher-risk findings usually move straight to biopsy instead.

• National Cancer Institute — Diagnosis and Staging
• The ASCO Post — Standardizing the Interpretation of PET Scans: An INR Equivalent