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PET scan vs. CT scan

mk1117's picture
Posts: 44
Joined: Jun 2006

I know this has been discussed somewhat in a previous post, but I still am questioning the differences. I've never had a PET scan, only CT scans. However, the CT scan I had initially right before my diagnosis didn't detect the colon cancer. The cancer was found through a colonoscopy, which was scheduled after my bloodwork showed extreme anemia. I haven't been feeling well, so my onc ordered another CT, which was clear. I also had a clear colonoscopy 7 months ago, but how do I know that the latest CT scan didn't show cancer in another area? Would a PET scan be more accurate in terms of seeing if there's cancer anywhere else?

I feel like I'm becoming a total worrywart - I think it's because I'm approaching my 5 year diagnosis date, plus I'm experiencing the same subtle symptoms I had before initial diagnosis. I have this fear that cancer is lurking somewhere, just taunting me because I'm so close to "cancer-free" status.

Thanks for your input!


PhillieG's picture
Posts: 4837
Joined: May 2005

I'm sure more people who know a lot more than I do about this topic will chime in, but from what I understand, the PET is more accurate because it measures how the cancer metabolizes sugar. You either drink or they infuse some "stuff" that has sugar in it, the cancer cells eat it up and it lights up on the scan. Gee, I hope I don't sound too technical ;-)

I know it's nerve wracking once you have cancer whether it's gone or hiding or what. I passed my 5 year mark a while ago and I'm still not NED but I try not to think about it much. Maybe if I hit the "all clear" mark I'd have a different view but I don't think so. Who knows.
I wish you good health and clear scans


Sundanceh's picture
Posts: 4408
Joined: Jun 2009

Hi Kathy

The CT scan is better able to detect masses...it can see the inner structures of the organs better.

PET does look for metabolic activity...a "radioactive" tracer is injected into you...you wait 45 minutes in a quiet room for the tracer to cycle through your body...the glucose in the radioactive tracer will be "eaten up" as Phil said by cells that indicate cancer. Normal cells do not "eat" the glucose in the same volume that a cancer cell does...so these areas "light up" and are an indicator of cancer activity.

I must add that you have to have NO SUGAR at least 24 hours before the procedure. In fact, try to stop eating altogether about 12 hours before. What this does is make sure that the blood gluclose level is below 120, so they can do the test. If you follow this, your reading would be somewhere between 92-110, so that would be good.

Insurance has to sign off on a PET scan, due to the cost of the test...it is outrageous.

With you getting clear CT scans and clear colonoscopies, your onc has not ordered a PET, beause it cannot be justified at this point.

Now, if you get a confirmatin of cancer recurrence, your onc could order a PET in addition to your CT and the insurance would approve it - but I know everybody's insurance is different.

The doctors use the CT scan to check for mass - the PET to check for metabolic activity and then they make their determination(s) from there.

Take care and nice meeting you BTW!

emrose's picture
Posts: 137
Joined: Dec 2008

from what I've been told and read, the CT scan detects a mass (in any organ they scan, not just the colon) and if a mass is detected, a PET scan would be done to see if that mass is cancer. This seems like standard protocol.

our oncologist did say that he prefers to do PET scans, but most insurance companies wont pay for it unless there is a reason - like something detected on a CT scan. I'd like to think that having cancer in the first place is reason enough for routine PET scans, but then again - what do I know!

I'm going to be calling our insurance company to see if they will approve a PET scan as follow up, but I have a feeling we'll be sticking to the CT scans.

I'm sorry you are worrying so much. I would say that if your CT scan and colonoscopy are clear then you are in good shape!!! Congrats on making it this far!

Posts: 232
Joined: Aug 2009

I am an 11 year cancer survivor. I had colonoscopies twice a year for the first 45 years I had a pet scan every year and MRI's extra blood work when I felt out of kilt and the doctors no everything is normal but normal for you is not normal for everyone else because 11 years ago the only statistic they could give me was a female my age 31 should not have cancer. That was it. So to aid them and to make me feel better we did it. Everything all the time when ever, what ever, because who knew since I should not have gotten it to begin with.RIGHT
See where I am going with this, no one knows you better than you and if it makes you crazy, uncrazy the day and request they do it. No harm no foul only great health care!
I am on this site as Goofyladie. Make it a Great Day!
Goofyladie (Cass)

Posts: 2
Joined: Sep 2009

Hi Kathy,
I am a new patient, and everything that has said to you it is absolutely correct. To begin, first you go for the CT scan and then if justify a PET scan follwos up; then insurance will cover for it, my did. CT scan detects masses in any organ and PET scan detect the cancer in any organ or part of the body. Cancer cells eats all the sugar and lights up to ease the view on the pictures taken. I just went thru this a couple weeks ago and I am undergoing on chemo and radiation at the same time for the next 5 weeks. Trust your doctors but I will recomend you a second opinion; I am. This can give you some peace and will help you to get your own conclusions and/or open more questions. I hope I've been helpful.


mk1117's picture
Posts: 44
Joined: Jun 2006

Thanks for everyone's helpful comments. I asked my dr for a PET scan and am having it tomorrow afternoon. Turns out my issues this go-round may be sinus-related. I just had a CT scan of my sinuses. My doctor's office & called said I had small "retention cysts" in one of my sinus cavities. Anyone else ever dealt with that? I see an ENT tomorrow as well. I'll be relieved after the appointments are over and I hopefully have some answers.

Take care, all.


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