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What is the difference between a PET Scan and a CAT Scan?

lmchils57's picture
Posts: 60
Joined: Feb 2010

I am still new to all this and yesterday I said something to my son about a PET scan and he asked what it was, I couldn't answer him. So questin is what is the difference between a PET Scan and a CAT Scan? He has had a colonoscopy and a CAT scan but not a PET scan so we are wondering if this is another test he needs.

msccolon's picture
Posts: 1956
Joined: Oct 2004

For one thing, I believe a PET is only done on someone who has confirmed cancer. It involves injected radioactive glucose, which is taken up by cancer cells at a higher rate than normal cells (hence some people's belief that if we could starve cancer cells of glucose, we could starve them, but our body converts its calories to glucose and that's what NORMAL cells require for life). It also can locate cancer that is a bit smaller than what will show up on a CT scan, although I don't know exact measurement accuracy. The PET takes longer, as the radioactive glucose has to move through your body for 45 minutes after injection in order to show up everywhere they want to scan. I am sure others will have more info for you as well.

geotina's picture
Posts: 2123
Joined: Oct 2009

A PET scan "lights" cancer up people refer to it, as an example, "mets lit up in the lung". I'm not sure how to describe it, someone with more knowledge of the words I'm sure will come on to help. For what's it worth, my husband has never had a PET scan, he has only had CT scans and he has only had 2, one at diagnosis and one after 6 months of treatment. George's CT scans were of the chest, abdomen and pelvis. Some get scans every 3 months depending on how they are doing I guess. From what the doc saw on the CT he didn't need anything further to confirm what he saw. Lots of people have never had a PET. Lots of people have had both. If you google PET Scan or log on the WebMD you will find out more. Take care - Tina

Kerry S's picture
Kerry S
Posts: 607
Joined: Dec 2009

A PET scan is a normal CT scan, but they also scan for radiation hot spots at the same time. You get the hot spots from the radioactive sugar water they put in you. Cancer loves sugar water and soaks it up faster then normal cells.

Then a computer program puts the 2 scans together. They call it a fused image.

robinvan's picture
Posts: 1014
Joined: May 2007

Hi Linda,

I wrote about PET scans on my BLOG a few years back and compared them at that time to CT scans. You or your son can find it here...
PET Scans
I've also included some links to other sites that define a PET scan.

I've had 3 PET scans and will be having another one shortly. They are often used in the early diagnosis stage to determine the overall extent of the cancer or at the end of treatment to measure results.

I'm a 5-year+ survivor. I've had about 2/3 of my liver removed.

I hope your son is coping well with treatment.

Rob; in Vancouver

“Medical science has proven time and again that when the resources are provided, great progress in the treatment, cure, and prevention of disease can occur.” Michael J. Fox

lesvanb's picture
Posts: 911
Joined: May 2008

Love the Michael Fox quote Rob!

Hi there

I'm a stage 4 rectal cancer survivor dx 5/08 one met to the liver resectable at dx. I had a PET initially 6/08 which confirmed the met in the liver. CT showed it too but couldn't confirm it was a met. I have been NED since my liver resection surgery (right lobe and gall bladder removed) 10/08. I am being monitored closely for 2 years with scans every 3 months because I am at high risk for recurrence which most often happens in the 2 years after surgery to remove all the cancer. Some docs say 2 years since the end of chemo which was 5/09 for me. I had a PET 8/09 and 12/09. Just had a clear CT 3/10 and will have another PET 6/10 and so on.


John23's picture
Posts: 2140
Joined: Jan 2007

"He has had a colonoscopy and a CAT scan but not a PET scan
so we are wondering if this is another test he needs."

PET scans aren't always as clear as a CT or MRI, and they often
indicate things that aren't a problem. They're generally used for
locating larger tumors or cancer cell clusters.

You can read about this stuff here: A Guide to Cancer Imaging Technologies

Subjecting a body to radioactive glucose isn't all that great a thing to do,
since "2nd cancers" (cancers caused by treatments) aren't uncommon.
Chemotherapy chemicals are all well known carcinogenics too, so I
suppose it's all part of the gamble...(?)

If the specialist doesn't feel a strong need for a PET scan, it isn't
anything to worry about.

But regardless, with any disease or invasive therapy, you are entitled
to a second and third opinion. It doesn't matter if the opinion is good
news or bad, there's always room for mistakes, and it's better to have
a few opinions to help sort out right from wrong.

Take care, and give your son our best!


Posts: 1956
Joined: Oct 2009

thanks, John......steve

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