CSN Login
Members Online: 18

You are here

how reliable are PET Scans?

bigman4christ's picture
Posts: 87
Joined: Oct 2012

I got the results of my PET and it showes 2 large masses in my liver. When we did a biopsy of one of the masses it came back benign, but the pet showes 2 large masses. I originally had 4-5 large masses in my liver, so is it possible to have some that are benign and some that are cancerous? what is the likely hood that there was a false positive reading?

John23's picture
Posts: 2141
Joined: Jan 2007

A quick read might do it?

"Limitations of a PET Scan
•PET scans can sometimes show up areas of high activity which may
be mistaken for cancers. Inflammatory conditions like rheumatoid
arthritis or tuberculosis absorb a lot of the tracer, and so can
cause confusing results.

•A PET scan is less accurate in certain situations:
1.Slow-growing, less active tumors may not absorb much tracer.
2.Small tumors (less than 7mm) may not be detectable.

3.High levels of blood sugar can cause the cells to absorb this
normal sugar rather than the radioactive, injected kind. Patients
are usually fasted for 4 hours before a PET scan, and blood sugar
levels measured to lower the chances of this happening.

•The radioactive substance has a very short decay and therefore
appointments must run on schedule.

•PET scans are a very expensive form of imaging, and are not
readily available. They often accompany other scans such as CT
and MRI in order to be diagnostically effective.

Results of a PET Scan:
After a PET scan, the doctor will receive an image of the patient
displaying the different amounts of activity present in different
parts of the body. An example of this is given to the right,
where areas of high activity in the brain are white, and low
activity are blue. If there is a cancer somewhere in the body,
this area will appear more active and so PET scans are often used
to see if a cancer has spread to other parts of the body or to
see how advanced a cancer is. A PET scan is not as accurate for
very small cancers however, so very early scans may not show anything."
From: http://www.virtualmedicalcentre.com

"What are the limitations of Positron Emission Tomography

Nuclear medicine procedures can be time consuming. It can take
hours to days for the radiotracer to accumulate in the part of
the body under study and imaging may take up to several hours to
perform, though in some cases, newer equipment is available that
can substantially shorten the procedure time.

The resolution of structures of the body with nuclear medicine
may not be as high as with other imaging techniques, such as CT
or MRI. However, nuclear medicine scans are more sensitive than
other techniques for a variety of indications, and the functional
information gained from nuclear medicine exams is often
unobtainable by other imaging techniques.

PET scanning can give false results if chemical balances within
the body are not normal. Specifically, test results of diabetic
patients or patients who have eaten within a few hours prior to
the examination can be adversely affected because of altered
blood sugar or blood insulin levels.

Because the radioactive substance decays quickly and is effective
for only a short period of time, it is important for the patient
to be on time for the appointment and to receive the radioactive
material at the scheduled time. Thus, late arrival for an
appointment may require rescheduling the procedure for another

A person who is very obese may not fit into the opening of a
conventional PET/CT unit."
From: http://www.radiologyinfo.org

Still confused?

Every test has it's place; each one has it's specific purpose, and
none should ever be used arbitrarily.

The one making the choice of tests to be provided, has the weight
of honesty and integrity on their shoulders; if that doesn't appear
to be the case, then that provider should be questioned.

In 2010 an oncologist sat at the foot of my bed in the ICU,
and scheduled a PET scan for later that week. I cancelled the
scan right after she left my room. When my surgeon made his visit,
I told him about the PET scan being scheduled, and he said
he would cancel it for me. When I told him I already did, he smiled.

He said I would have "lit up" like a Christmas tree; that it was
way too soon after surgery to have a PET. There were too many
cells in the healing process consuming glucose at a fast rate for
there to be any serious accuracy.

It's great to find an honest practitioner, isn't it?

Best wishes for better health,


bigman4christ's picture
Posts: 87
Joined: Oct 2012

Thanks John! it is confusing but i am understanding it better :) i believe that we/I did everythng that i was supposed to and so did the Dr. so i am feeling pretty confident, that i did everything i needed to do for the scan. I really like my Dr. and trust him so that is good. He was also a little confused about the readings so he is actually meeting with a board today to run this by them. I am very happy that he is willing to take advice from other people that might have different opnions than he does. I know that he is not new at this because he has been doing this for 26yrs and he is the director of the department so i trust him. that is kind of funny that you had to cancel your PET right after she scheduled it for you. defenitily not a cheep scan that can be done just because. thanks again for the info!

Helen321's picture
Posts: 1346
Joined: May 2012

Scans can be unreliable and so can people. I learned that the hard way. I trusted my doctor and trusted the scans. I went from stage 1 to stage 3 most likely either a doctor's error or an unreliable scan. My new surgeon at Sloan suggested it but couldn't say it out loud. Could have been anything, wasn't visible yet, I'll never really know. First doctor practiced 23 years and was head of department. I've learned head of the department can simply mean you speak well at conferences. He wanted to give me my second surgery without chemo or radiation which is stand protocol.If you are in a cancer hospital, that is good because they usually have a team and so you'll get a varied opinion. The only thing is they refused to do another PET at the second hospital, felt it wasn't necessary. I still disagree. Who's to say the first PET was accurate now. If you aren't in a hospital with a team, please get a second opinion in another hospital, it's really worth the time if you have the insurance to cover it. I have 4 doctors on my team at Sloan. If I don't like what one is saying, I can ask another for his opinion. They do contradict each other sometimes which is good. It means they are being honest and not just agreeing with their partners. When they have a team meeting about the disagreement and there are varying opinions, they call in visiting doctors from sister hospitals or other teams for their opinions. Helen

bigman4christ's picture
Posts: 87
Joined: Oct 2012

Thanks Helen, that is good advice. it seems to be a unanomous opionion from everyone that we need second opionions and do research! :) I will have to get another opinion it sounds like. I am at the cancer hospital so there are several Dr. close by.


annalexandria's picture
Posts: 2573
Joined: Oct 2011

has been that the PET is extremely sensitive, and has always picked up my tumors while they were still pretty small, while unfortunately also giving some false alarms. I've had several spots that lit up that didn't show up on later scans (I wasn't having any tx at the time) so apparently were not cancer. But I'm willing to put up with that problem, as the CT alone missed significant tumor growth, which led to a lot of unnecessary chemo and a really lovely near-death experience. It's taken a certain amount of battling with insurance, but I now always get the PET/CT scan, which has enabled my surgeon to jump on tumors quickly as they develop and get them the heck out of there. I really believe that this is what has kept me alive well past my expected prognosis and led to a NED scan in March. Can't remember if you mentioned it...what were the SUVs for the spots in your liver? That would give some suggestion of how active the cells are in each area of concern. Hugs! AA

bigman4christ's picture
Posts: 87
Joined: Oct 2012

Hey AA, I am still trying to learn all of the lingo and abbreveations, besides a 4x4/car what does SUV stand for? and does NED stand for No Evedince of Disease?

annalexandria's picture
Posts: 2573
Joined: Oct 2011

and SUV=standardized uptake value. Basically this shows how active the cells are in a particular area. Some parts, like the bladder (esp if you have to pee), will have a high SUV normally, but generally the numbers are pretty low for normal tissue, while cancer will have higher cellular activity and thus a higher number. There's no hard and fast rule, but in my experience it seems like things under 3 or so TEND to be non-cancerous, while higher numbers TEND to be cancer. My tumors have all been in the 4-5 SUV range, but I also have a spot from my last PET that glowed at 7.0. No corresponding physical structure on the CT, so my doc thinks it's just an anomaly. It's confusing, so very sorry for throwing out the technical terms without clarification. Have been doing this for a while now, so forget that some are new residents here at Camp Cancer and still have to learn the secret handshakes! Anyway, I like to know what my SUVs are, just to get a sense of how active things are in each area. Hugs~Ann Alexandria

petech's picture
Posts: 2
Joined: Jun 2017


Hi. I am a Pet tech/marketer for Pet scans. I wanted to put in my two cents here. :)


Most of the time your doctor will order a CT or, it has happened very frequently the CT was ordered for something else but something showed up on the CT. A doctor has really, three choices. Send that patient for a Nuclear Bone scan, send them for a PET scan or perform a biopsy.


A Nuclear Bone scan will show lesions on bone. The dose injected is referred to as an isotope and is different than what is used in PET scans. The radioactivity in the isotope is low energy (173 keV). The binding agent is also different. A bone scan is to look at the skeleton. It is not a good option for picking up tumors or tiny cancer cells.


PET (Positron Emission Tomography) is a high energy isotope. (511 keV) As the name implies, it uses a positron that emits its energy through annihilation. The amount of isotope is not harmful in small doses. The dose you receive is very minimal. Just enough to get what we need.


The binding agent with the positron is glucose. Yep. Good old fashioned glucose. In nuclear medicine we wish ALL of our scan were a one shot one kill type of scan. We managed that with the I-131 and I-123 capsule for determining thyroid abnormalities and tissue kill. Unfortunately, as awesome as modern medicine is, PET imaging is at the forefront of its time in cancer detection.


To have an excellent scan requires cooperation of the patient as well. It is absolutely necessary to refrain from strenuous exercise the day or two before your scan.  Your muscles take up glucose as food in your regular diet. This isn’t any different. If you have exercised, then your muscles will be looking for food anyway it can get it. The problem is that areas of concern will be impossible to detect because all parts of your body are taking up the glucose which is tagged with the isotope.


It is also important to ABSOLUTLY not have anything except water to eat or drink for 6 hours prior to your scan. Eating a low carb diet prior to that is most beneficial as well. (Anything to prevent the competition of uptake.)


Finally, if you are diabetic, insulin will affect the scan. You will need to let the facility scheduler know what type so that they can give you instructions. Really, if you forget…cancel and reschedule. These are critical to the best scan you can get.


The benefit of the PET scan is simple. Prior to PET, the next choice in detection was biopsy. PET (although it is not 100% accurate) is much less invasive as even though it IS what some would considered expensive…it is far cheaper than a biopsy. My personal thought is this is why insurance companies fought against it so hard. There are still insurance companies that will not acknowledge PET as a diagnostic tool. PET can determine possible cancer through an injection and a 30-45 min scan with a very acceptable percent of accuracy. Used in combination of the biopsy, it has proven to be a very useful tool in combating cancer.


Although we admit it isn’t full proof…our fight without it is not nearly as good.


I hope this answers your questions beyond what you can find on the internet.  


Trubrit's picture
Posts: 3727
Joined: Jan 2013

I wish Bigman4christ were alive to read it. He passed away on his 30th birthday, several years ago now  

This would make a great stand alone post  






mendy3233's picture
Posts: 5
Joined: Feb 2017

Can cancer show up right after Pet/ct, I have had throat cancer .Had radiation and chemo. Pet/ct shows all good .but I now have growth on tongue,2 weeks after  test.

Subscribe to Comments for "how reliable are PET Scans?"