PET scan vs CT scan

[Patty F]

I know that I have read on the boards that PET scans are better to see some things and CT scans are better to see other things. Can someone please let me know which is which? I promise this time to save it so I won't have to ask again. (Dang chemo brain)

[Yorkiegirl]

Patty I have had the combo one PET/CT where both are done.

[SoCalGal]

Hi Patty-
They do a PET/CT which is mostly a PET with a 5 minute CT enhancement during the PET. With contrast during the CT part. I prefer the CT at the very end. My onc's office does the CT at the very beginning so I have my scan at the hospital where they do it my way!

[Joan M]

Hi Patty,

Glad to see that you're doing well and can't blame you for wanting to the best follow-up you can.

PET/CT and CT scan are really two different tecnologies.

Each has it's good and bad points and sometimes it can be hard to determine which is best.

The CT uses a contrast medium and can "see" nodules as small as 2-3 mm, for example, (there are 10 mm in 1 cm, and 1 cm is about 0.34, or about a third of, an inch). On the other hand, although the CT scan can see a nodule that small, it might not be able to identify it as benign or malignant. However, such a nodule can then be followed. (In the case of cancer, a CT scan can be done for the chest, abdomen and pelvis, for example. They're done in one setting, but there will be three separate reports.)

On the other hand, a PET scan doesn't really "see" anything but rather measures metabolic activity of glucose, which rapidly metabolizes in cancer (and in other types of) cells. A "nodule" will "light up" on the screen at about 1 cm to be sure if there's a potential problem, and the probability of it lighting up below that size, even if it's malignant, decreases as the size of the nodule decreases (as in a linear relationship).

The PET is a nuclear test. It does not use the same "contrast" as a CT scan, which uses iodine. The PET instead uses a nuclear isotope (FDG 18, or something like that). Sometime a reports states, "no FDG avid areas," if nothing lit up.

The CT had been eventually added to the PET to give position to the whole PET scheme. It adds a better view of the liver, lungs, etc. to better pinpoint any problems.

Which is better?

Case in point (you decide): In May 2006 I had a PET/CT, which is whole body for everybody (meaning the organs and bones from the upper neck to the middle of the thighs). The scan was clean.

In May 2007 the scan was repeated and it revealed a 9 mm malignant nodule in the left lung which was eventually removed by a wedge resection. It recurred a year later in April 2008 as residual from the surgery and I had radiofrequency ablation to attend to that this past August.

When the 9 mm nodule was noted in 2007, my onc asked the facility where the PET/CT was done to determine whether that nodule had been there in the PET/CT of 2006, and it was, as a 5 mm nodule. But because the scan was a nuclear scan it was too small to light up and therefore it was never noted.

It's difficult to say whether this was standard of care, meaning that perhaps another facility may have noted the nodule in the CT part of the report even though it didn't light up on PET.

Joan

[freyja]

So what kind of scan is used to make sure the brain is clear? I've had MRI's of the torso, CT scan and bone scan. I'm worried they haven't really checked the brain.

[Rich66]

Joan,
Not noting something which was noticeable does sound like a poor practice. Seems like it should have been cause for a CT if the CT part of the PET/CT was equivocal. But..not noting it????

[Chelee]

Freya, As to the brain I would request a MRI. Some oncologist will try to talk you into a CAT scan. (It's cheaper.) I would not be happy with that.

A MRI of the brain is much better and since we are dealing with her2...that is the only acceptable scan in my opinion. I like to have a brain MRI at least yearly. (If you've never had a baseline MRI of brain...nor one since you finished chemo I would probably mention this to your onc.)

Most oncologist won't do scans unless you present with symptoms. That's probably why your onc hasn't done an MRI of your brain yet. You could ask your onc about this if your worried. I've been having bad headaches and I have a MRI of brain in a week.

Chelee