On a pet scan, are the dark areas cancer?

It depends: The pet scan is shown on a computer, so what's light and what's dark depends on how it is set up. Look at the picture on the right. The first image shows areas of possible cancer as dark; the second image of the same patient shows the reference ct scan, and the last image has added color so the cancer areas are red. The brain always "lights up" on pet scans because it uses so much glucose!

Dr Turrisi agreed

Yes and no: Areas with cancer are usually hypermetabolic in a pet scan due to the increased uptake of the radioactive glucose by cancerous cells. Looking at an FDG pet scan alone, you can grossly see "dark" areas in the brain, heart, kidneys and bladder and these are due to physiologic distribution/excretion of fdg. Not all "dark" is cancer and cancer is often but not always "dark".

Like all images, PET: Gives a perception of truthm but onlt biopsy proves cancer. Increased metabolism (dark/bright) areas occur in inflammation, and sarcoid produces some very hot readings, but not cancer. Slow growers, like broncheoalveolar often are cancer but rarely glow on pet. And the scans are hugely expensive.

Not always: On a pet scan many normal areas may look dark. Many of these are normal. For example urine, the heart and other areas appear dark but are normal physiological uptake. This is why a specialist must read the scan to determine what is normal and what is true hypermatbolic activity or cancer on the scan.