PET stands for positron emission tomography, and this generally uses a safe radioactive tracer molecule called 18F-fluoro-2-deoxy-D-glucose (FDG). Fortunately, that’s not on the quiz — you don’t need to know it. All that is worth knowing is that PET scans offer “metabolic imaging”, which detects differences in the metabolism of tissues. The most metabolically active tissues have the greatest needs for sugar from the bloodstream, and when the sugar is labelled, the scans detect these areas as “PET-avid”. The objective measurement is a number called a “standard uptake value”, or SUV, where higher numbers mean a higher metabolic rate. While metabolic activity can be increased with inflammation, infection, and normal body activities (organs like heart, brain, and bowel have uptake normally from regular ongoing activity — this is NOT a bad thing) the reason we all care is that PET scans tend to pick up cancer, because cancer cells generally have greater metabolic activity and are dividing faster than most normal tissue.
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Some slower growing cancers, such as bronchioloalveolar carcinomas(BAC), are much less consistently reported to appear on PET scans than other NSCLC tumors. And there is some evidence that higher SUV activity of lung tumors correlates with faster progression and worse prognosis.
Now, the first area where PET scans earned their place in oncology is in initial staging of cancer. This is very routine now, and it’s clear that CT (for computerized tomography, but it’s always referred to just a CT or CAT scan) is very good at assessing shape and size of organs and nodules. However, if a lymph node is not enlarged or there’s a spot in the liver that has the same density as the regular liver tissue, or there’s some other spot that looks like it may be a benign cyst, CT scans have a hard time detecting cancer. They’re great for size and shape of things, but they can’t say if something is very metabolically active or not. PET scans are great at determining whether something is metabolically active or not, but they generally can’t pinpoint size very well. Also, the SUV needs to be considered relative to the size of the lesion. A nodule of 1 cm, or these days probably down to 6-8 mm, can likely be detected on a PET scan, but a tiny 2-3 mm nodule probably doesn’t have enough cells, even if they’re very metabolically active, to light up on a PET scan. So PET scans use a combination of how metabolically active the cells are, and how many there are in an area. They don’t tend to be very reliable at determining whether pleural fluid has cancer cells, because those cells aren’t concentrated into a metabolically active cancer ball (not a technical term), but are diluted in fluid that is not very metabolically active.
So CT scans and PET scans both have strengths and weaknesses. We can get useful information by having patients get both types of scans and holding them side by side, seeing if an ambiguous lymph node or spot in the liver on CT is “hot” on PET. That’s good, but over the last few years, the newest machines are now able to perform a fused PET and CT scan and overlay the results on top of each other, so that you can directly see what area in the chest or abdomen is lighting up on PET (note also the heart lighting up red, which is just normal activity):
Without the PET, the slight thickening on the left chest wall would be pretty questionable, but seeing that it’s also metabolically active on PET makes it nearly certain that this person has progression of her cancer inside the chest wall (she had previously had this found at surgery, and then responded well to chemo afterward, with no residual nodules on her scans until this one).
On the other hand, we still have plenty of situations where even with CT and PET scans, or a PET/CT fusion scan, we can’t figure out what is residual cancer, what is inflammation after treatment (particularly radiation or chemo and radiation), and what is infection. This scan is of a patient who underwent chemo and radiation for a stage III NSCLC, and months later we see some collapsed lung and a lot of scarring, but we can’t tell where inflammation stops and cancer begins (heart again lighting up in center of picture):
This is worrisome, but after chemo and radiation we often see very ambiguous changes that even can light up on PET and then subside over time.
Overall, we’re still learning more and more about the value of PET scans and PET/CT fusion scans in following patients over time. At my own center, we actively debate whether the added information we gain from post-treatment PET scans is reliable enough to incorportate into treatment decisions. New technology can be remarkably promising, but we are struggling to have our knowledge and experience catch up with the capabilities of the new imaging machines. In a few subsequent posts, I’ll go into more details on emerging data for using PET scan imaging in following patients on or after treatment.
posted by Dr. West @ 6:15 pm link to this post
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January 9th, 2007 at 7:19 pm
Dr. West,
I have an appointment with you on 1/18. I just completed both a pet/CT on Monday 1/8. Got radiology report today and somehow radiologist made no mention of secondary lesion in left lung that last measured 2.1 X1.1 cm. I will fax your office this radiology report as soon as I get this omission corrected. Slight growth in primary left lung lesion after 4 maintainence cycles of avastin. Primary lesion had reduced in size after 3 cycles of carbo, taxol and avastin and was stable after 6. I have most of my CT and pet scans on CD and could fedex you a copy before appointment if that would be of value. Pet scans are jpg with an avi thrown in while CT scans use DICOM viewer. Of course I do not know how to interpret these scans, but being a MAC user OsiriX seems to be a better viewer
January 9th, 2007 at 8:04 pm
Happy to get FedEx copy of CD, and also report. Will review.
-Dr. West
January 10th, 2007 at 7:18 am
Thank you for your very interesting reading on Metabolic Imaging: How PET Scans Are Changing Oncology
My husband just had his 3 month PET scan after remission of 1st treatment of SCLC. The PET showed several areas of hot spots indicating bone metastisis. He then has had a bone scan, which showed no bone mets. Confusing, but consistent with you article. Thank You. lban
January 17th, 2007 at 4:04 pm
My husband was diag with LC 5/05 10cm mass nscl ( non smoker) he had his upper right lobe taken out and no lymph nodes were involved and had four months of Chemo to be safe.He goes in for scans every three months and this last PET scan showed an uptake of 4.6 in his mid abdomn upper right lobe. The pet report said to have a ct scan of the abdomn but his onc said he should go for a colonoscopy.I would think a CT scan would be better. His last pet scan in 2006 didn’t show this. Hubby said he has had enough but we are going to the gasto doc today. Words of advice please.Also my husband also has a low level uptake in the area of the vocal cord region and is un changed since 3/06 and with an ultra sound and labs last year they are not worried since those tests show nothing wrong. My question is would a colonoscopy show anything since the mid abdomn’s right lobe is to high for a colonoscopy to see anything.
January 17th, 2007 at 9:03 pm
Lorrie,
This is a bit too specific of a question for me to tackle. I don’t have enough details and I can’t replace the advice of your physicians. The colon often shows some degree of normal “physiologic” uptake, as I indicated above. A colonoscopy does a pretty thorough job of investigating problems in the colon, so I imagine they’ll be able to clarify whether there is any need for additional work-up.
As I mentioned above, PET scans show all sorts of things that range from normal function to cancer. The people who read PET scans are specifically trained to do that, and I haven’t had that training to give a good interpretation even when I have the images in front of me, so there is absolutely no way I can provide insight about how to interpret a PET scan I’m only hearing about without any details. I can’t offer specific medical advice on how to manage your husband’s situation.
-Dr. West
January 18th, 2007 at 10:37 am
Thank you Dr West.Our ONC called us back last night and Gary will have CT scan as suggested by the PET scan report on his midabdomn.Thank you again
Lorrie
May 15th, 2007 at 7:25 am
Hi Dr. West. Thank you for taking the time to write this article about PET scans. Following a bronchoscopy, I was diagnosed with BAC about 2 years ago. I had a 5 cm tumor in my right upper lobe but a PET scan taken after the bronchoscopy and before the bilobectomy was negative. But I just had my second PET scan last week. This one was done by a different facility and in a slightly different manner. I was on a low-carb diet for two days prior to the scan, in addition to fasting the morning of the scan. A CT scan was also done at the same time. The report states there is a “1 cm right suprahilar node near the right upper lobe stump has been present on exams dating back to 5/23/2006, but has an SUV of 5.4. A small right hilar lymph node measures approximately 4 mm short axis and has SUV of 3.2.” Where is the suprahilar? What recommendations would you make at this juncture? I will see my onc on Thursday to discuss.
May 15th, 2007 at 4:59 pm
Suprahilar is in the upper mid-chest part of the lung. The hilum is the part of the lung that’s near the branching point of the windpipe, away from the outer edges near the chest wall. Supra means above, or at the top part of the hilum.
Those numbers suggest that there is some viable cancer in very limited areas, but the lack of change over a long time suggests that this is likely a very indolent process. As I’ve indicated in other places on the site, I would not be “trigger-happy” enough to change an otherwise very effective treatment approach (including no treatment, just watch and wait) on the basis of PET uptake, particularly in the absence of progression on CT. If something is that subtle that it can’t be detected as progression on the more validated measures used in oncology, I would be very reluctant to make treatment changes based on it.
-Dr. West
November 3rd, 2007 at 9:25 pm
[…] We know PET scans can provide additional metabolic information that can be more sensitive and specific for cancer than chest x-rays and even CT scans in the initial staging of lung cancer (see prior post on introduction to PET scans). PET scans are now nearly universally employed in the initial workup, at least of patients who have NSCLC and aren’t already known to have stage IV disease. But how useful is this technology in the setting of surveillance for the patient at risk for recurrent/residual disease after curative treatment? […]