An FDG PET scan uses a radiotracer to measure things like blood flow, oxygen use and sugar metabolism in your body. It shows how your tissues and organs are functioning at the molecular level. For cancer patients, PET scans are used for cancer staging, monitoring treatment or to check for recurrence.
When you read results from a PET scan, you may notice a number called a standardized uptake value, or SUV. But what does it mean? And what does it mean if those numbers change?
We spoke with diagnostic imaging specialists Seth Gammon, Ph.D., and Lesley Flynt, M.D., about what cancer patients should know about SUV numbers and how they help your care team make decisions about your diagnosis and cancer treatment.
How do PET scans show cancer?
A tumor is something that grows uncontrollably, and that growth requires energy sources. One of the fuels that tumors use is glucose, a type of sugar. PET scans can show doctors where a lot of glucose is being used in the body compared to surrounding tissues.
Before a PET scan, you’ll be asked to fast for a few hours so that any glucose you may have ingested by mouth won’t compete or interfere with the glucose that is injected for the scan. The imaging tech inserts an IV injection to give the tumors glucose, then the images will light up where the sugar is being used.
“Whenever you see a spot on a PET scan using up lots of sugar, you can assume a tumor is growing or there is active inflammation in that spot,” explains Gammon.
It’s important to carefully follow your care team’s instructions before a PET scan. Fasting and how much you move your body can impact the imaging results.
“The SUV number can vary from the time between the injection and the scan based on how each individual metabolizes glucose,” says Flynt. “We recommend sitting quietly for 60 to 90 minutes after injection for accurate results.”
Gammon cautions that while oncologists use glucose to measure cancer growth, cutting sugar out of your diet completely will not starve the tumor.
“Sugar is just one of the fuels for tumors. They can switch to other fuel sources, and they do,” he says. “Your brain also runs on sugar, so you can’t just cut off sugar consumption to the body.”
What do SUV numbers mean?
The images created from PET scans have bright spots and dark spots. The measurement of brightness is the SUV number. In general, higher SUV numbers may indicate a malignancy, or cancerous growth.
“Think of an SUV number as a tape measure for radioactive activity in the body,” says Gammon. “It’s a continuous measurement from zero to infinity. But there are practical limits to that.”
SUV values will have a different meaning depending on the cancer type. Your care team will be able to explain what the number means for your specific cancer.
How are SUV numbers used to make cancer treatment decisions?
When looking at SUV values, it’s important to know that cancer is not the only thing in the body that takes up glucose. Inflammation, infections and autoimmune reactions can also create bright spots. In those cases, the SUV value is likely to be the same throughout the entire organ, instead of only one spot on the organ.
Flynt says an important part of understanding PET scans and SUV numbers is to look at background activity. For example, you can see a high uptake in SUV numbers in the larynx, or vocal cords, because of humming or talking, even if there is no tumor activity there.
“When I’m looking at a first PET scan for a patient, I’m noting the baseline of how active their tumor is,” says Flynt. “During initial staging scans, I’m specifically looking to see where the primary cancer is and looking at the rest of the body to determine if there are metastases.”
SUV values can have a big impact on your doctor’s treatment decisions, says Flynt. Oncologists use your SUV values to determine if a therapy is working or not. Flynt says SUV numbers often change before there’s a noticeable change in tumor size because the changes in metabolism occur faster than changes in anatomy.
In follow-up scans, a decrease in SUV numbers can indicate that the current cancer treatment is working.
“For example, if an SUV number in a lymphoma patient’s tumor goes from 7 to 2, that can tell your doctor to continue with the current treatment plan,” says Flynt.
Gammon says he hopes PET scans and SUV numbers can be used even more in the future to further personalize cancer treatment and care.
“We probably do more PET scans at MD Anderson than anywhere else in the world,” he says. “We’re always researching ways to make this technology work toward better treatments and prevention.”
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