Tumor Measurement CT vs. PET

[Bill]

I've asked a question about this on the Ask The Experts board. If anybody else is familiar with this happening please provide your explanation here.

Thank you.

[Fay A.]

The difference in size was explained as dead tumor tissue showing up on CT and only the active malignant tissue showing up on PET. On subsequent scans the dead tissue was resorbed and what was previously active malignant tissue showed no viable tumor on pathologic exam. This change was thought to be a result of treatment effect (Iressa). Hope this helps.

It takes time for the body to clear away the dead tumor.

[john]

As Fay said a CT only measures the size. A CT scan could show a mass but it might be dead or benign. There are other things that it shows such as whether a mass is calcified or non-calcified. Non-calcified is more likely to be malignant from what I have read but sometimes calcified can be too.

A PET scan measures the metabolic activity. When you take a PET scan, you are given basically sugar. Since cancer cells usually show higher sugar intake than normal cells the cancer "Lights up".

The measurement on a PET scan is called the SUV (Standardized Update Value). Greater than 4 (I think) is when things have to be followed up. But there are parts of the body that always have a higher SUV like the brain.

There is a new scanner - a PET/CT scanner that combines the two. This way both can be seen on one scan versus trying to compare two different scans

[jcawork]

Sometimes on PETs they make the mistake of attempting to measure and report these in the report. . They can't as this is about light measurements and there as "aura" that makes anything on a PET measure larger no matter what the SUV. CT measures more accurately. PET is good for SUV only and thats not always malignant. But if something doesnt light up, its usually not cancer.

Jen