What does 'hypermetabolic' mean on a PET scan?

The clause: 'Hypermetabolic 1.4 cm right hilar lymph node.' A PET/CT is two scans taken in one sitting. The PET half shows where the tracer went. The CT half shows the body itself, so the size (1.4 cm) and the place come from the CT. 'Right hilar' means the root of the right lung, where the airways and blood vessels go in, and where lymph nodes normally sit.

Read the two halves together. Brightness says how hard the cells were working. The CT says what is there: how big the node is, what shape it is, what sits next to it. A bright node the CT calls ordinary and a bright node that has grown are different conversations, and the oncologist weighs both halves. If the report adds 'clinical correlation recommended', that's the radiologist handing the question back to the doctor who knows the patient, and what that phrase asks for has its own guide.

The scan also has blind spots. PET resolves detail down to about 5 to 7 mm, so spots under 1 cm are often missed or look fainter than they are. High blood sugar on the day of the scan can make spots harder to read too. The opposite word, 'hypometabolic', means a spot took up less tracer than its surroundings.

The clause: 'SUVmax 6.2.' SUV stands for standardized uptake value. It compares how much tracer sits in a spot with how much would be there if the dose had spread evenly through the whole body, adjusted for body weight. An SUV of 1 is that even share. An SUV of 6.2 means the spot held about six times its even share. Some reports use SUL instead, the same idea adjusted for lean body mass.

'Max' means the single hottest pixel in the spot (radiologists call it a voxel). It's the number most reports print, and it's also the jumpiest, because one pixel is easily nudged by image noise. You may also see SUVpeak, the average over a small sphere about 1.2 cm across on the hottest area, or SUVmean, the average over an outlined region. 'Liver SUVmean 2.1' is that kind.

No single SUV means cancer. The number you'll see quoted is 2.5, traditionally used to call a lung nodule cancerous. It holds up poorly. In one 2008 study of 173 people with lung nodules, among nodules 1 cm or smaller the 2.5 cutoff caught 85% of the cancers but correctly cleared only 36% of the harmless ones, and it was right 54% of the time. For nodules over 3 cm, it cleared just 17% of the harmless ones.

The reason is that sugar use isn't specific to cancer. Low-grade cancers can sit at a low SUV, and inflammation or infection can run high. A brighter spot is generally more suspicious than a faint one, but no number decides on its own. That holds for an SUV of 4 and an SUV of 15 alike: each is read against the liver, the CT and the last scan. On this report, 6.2 is about three times the liver's 2.1, and step 3 covers why the report gives you the liver at all.

The clause: 'Liver SUVmean 2.1.' It looks like a throwaway line. It's the yardstick. Every body takes up tracer a little differently, and every scan runs a little differently, so radiologists measure a spot against tissue in the same person on the same scan. The two standards are the liver and the blood pool, meaning the blood in the large vessels in the middle of the chest (the mediastinum).

Under the European guideline for FDG PET scans, a normal liver reads about SUV 1.3 to 3.0, and the blood pool about 1.6. The example's liver, at 2.1, sits inside that range, so the yardstick itself looks ordinary. The node was 3.1 last time, just above the top of the normal liver range, and it's 6.2 now, well above it.

That's how to hear words like 'mild', 'moderate' and 'intense' on a PET report: brighter or dimmer than this background, in the radiologist's judgment. They aren't fixed numbers, so there's no chart that converts them. Spots next to organs that are bright by nature, like the brain, heart, kidneys and bladder, are also harder to judge, because the background there is already high.

One scale turns this comparison into a score, and you'll meet it if the cancer is lymphoma. The Deauville score runs from 1 to 5: 1 is no uptake above background, 2 is at or below the blood pool, 3 is above the blood pool but at or below the liver, 4 is moderately above the liver, and 5 is markedly above the liver or a new spot. A 1 or 2 counts as a complete metabolic response. A 3 often means a good outlook, though some trials count it as not enough. It's built for lymphoma, and nothing in the example says lymphoma, so don't score it.

The clause: 'Physiologic activity in the brain, myocardium, kidneys and bladder.' Physiologic means normal, the body doing its ordinary work. Myocardium is heart muscle. The radiologist lists these so the doctor knows the bright areas were seen and judged expected.

Normal uptake shows up in more places than people expect: the brain, the heart (which varies from scan to scan), the kidneys and bladder as the tracer leaves the body, muscles (more after exercise or insulin), the bowel (more on metformin), the tonsils, the thymus in young people, and brown fat, a heat-making fat that switches on in young people and in the cold. Bone marrow can light up after growth-factor shots given to raise blood counts, and with infection, inflammation, anemia or chemotherapy.

Then there are bright spots that are real but aren't cancer. Infection, inflammation, tuberculosis, fungal infections, sarcoidosis, lung inflammation after radiation, and healing after surgery all take up FDG. In one review of these false positives, brown fat in the neck was reported as a false positive in 2.3 to 4% of patients.

Vaccines matter too. In 2021 guidance on mRNA COVID vaccines, swollen lymph nodes were reported in up to 16% of patients, and on a PET scan they can light up in the armpit and low neck on the side of the shot for 4 to 6 weeks or longer. So tell the care team about a recent vaccine in that arm, a recent infection, surgery or procedure, radiation, and any growth-factor shots. The example's node is in the chest, not the armpit, so the vaccine point doesn't fit it, but a recent chest infection would be worth mentioning.

The clause: 'previously 3.1.' This is often the line that matters most, and the easiest to over-read. Two SUVs only compare fairly when both scans were done the same way. The European guideline asks for scanning to start about 60 minutes after the injection (55 to 75 is acceptable), and for a repeat scan on the same scanner, with the same image settings, within 10 minutes of the first scan's timing. PERCIST, a research standard for measuring response, allows 15 minutes and the same scanner or model.

Even then, numbers wobble. PET measurements from two scans of the same tumor vary by as little as 6 to 10%, and variability up to 42% has been reported. A move from 3.1 to 3.3 can sit inside that wobble. A move from 3.1 to 6.2, a doubling, is far bigger than it.

Researchers also set a bar for when a change counts: under PERCIST, a partial response is a drop of at least 30% and at least 0.8 units in the peak value, and progression is a rise of more than 30% with more than 0.8 units, or a new spot typical of cancer. It's a yardstick for trials, not a rule for reading your own report, but it shows how big a change has to be before experts trust it. 'New' is the other word to watch. In PERCIST and on the lymphoma scale alike, a new spot counts on its own (in PERCIST, one that looks like cancer rather than infection or treatment effect).

The same clause reads very differently when the news is good. 'SUVmax 2.4, previously 6.0' means the spot has calmed down. 'No longer seen' means it no longer stands out from what's around it. 'No FDG-avid disease' means nothing on the scan took up more tracer than expected, and 'complete metabolic response' is the radiologist's call that the active tumor has disappeared from the images. That call is made by eye, not by a number. It's the best line a PET report can carry, even though a very small spot can still sit below what the scanner sees.