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Reliability of PET for diagnosis

Posts: 3
Joined: Jan 2020

Hi. Had stage III-A colon cancer and finished chemo in mid-2017. December CT showed a 2x2.5cm mass in spleen which doubled in size since previous December. Have PET scan on Tuesday.

a) is that considered rapid growth? If not, what is?

b) any info on false positives/negatives on cancer diagnosis using PET scan? Did some googling and got scientific articles over my head.


Trubrit's picture
Posts: 5087
Joined: Jan 2013

Sorry you find yourself here, but it sounds like you've been on the journey for a while now. 

I cannot answer your quesiton about rapid growth, and all I can say about PET Scans is from my own, personal experience, but if it helps a bit, hey, I will feel good. 

When my CEA started to rise after treatment and one good result, I was sent for a CT Scan, which showed nothing abnormal. My Oncologist then ordered a PET Scan which showed a 2 cm tumour in my liver. 

So for me, the PET was more accurate than the CT at that point.  I have had one other PET Scan since then, and all was well, so I cannot comment any further. 

From what I have read, PET Scans are more accurate than CT's in some cases - as in mine. 

I think there is always a chance of false positives with any kind of test.  

Hopefully someone with more knowledge will come along soon and answer your questions. I basically just like to say hi and welcome. 


Posts: 3
Joined: Jan 2020

Thanks a lot Tru!

beaumontdave's picture
Posts: 1036
Joined: Aug 2013

I had both that sort of growth in a year's time, and false positives, at my neck and armpits from a PET scan, so yes these things happen. If it helps, know this 111b had it hit my liver twice, and i 'm here, and NED for over 5 years, 12 years past diagnosis...............................Dave

Posts: 3
Joined: Jan 2020

Good for you, Dave! thanks!

Posts: 247
Joined: Dec 2013

Saying one scanning type is better than another is a bit too simplistic.  Each scanning technique uses a different method of imaging or detection.  CTs and MRIs go about trying to provide a visual/physical image.  CTs use ionizing radiation much like X-rays to create a 3D image of internal structures of your body.  The use of contrast medium helps to enhance features.  MRIs use radiowaves to do the same and contrast medium is used to enhance internal features.  Both require a radiologist to visually examine each slice image to detect any abnormal features.  This is why things can be missed and why one radiologist/doctor can have a different conclusion than another.

PET scans use a different mode of detection.  For cancer, it leverages the fact cancer cells are hypermetabolic.  This means cancer cells consume more energy than normal cells.  Why this is important is because of how the PET scan detects cancer cells.  A radioactive glucose tracer is injected into your body.  Cancer cells will uptake more of this radioactive glucose than normal cells.  This higher concentration of the radioactive tracer will be picked up by the scan machine as a hot spot.  The measure of the amount of uptake is called SUV.  Typically SUV uptake of more than 4-5 is indicative of cancer cell activity.  But this is not a total guarantee as there can be false positives.  Many times a biopsy is ordered as a follow up confirmation for any potential cancer diagnosis.  This is why you're told not to exercise or exert yourself the day before your PET scan and that you are to go on a specific diet.  The reason being they want to lower the "noise" floor of your body to ensure any possible cancer activity is picked up.  The exclusion of exercising and physical exertion is to prevent increased non cancer uptake of the radioactive tracer.

Posts: 6
Joined: Jan 2020

These days the centres are doing PET/CT scan,which is kind of superimposition of the pet scan on the ct scan,. The oncologists prefer the pet scan and base most of their diagnosis on the basis of the pet/ct scan. Still biopsy is the most certain way to know if any kind of mass has cancerous cells and their nature. But at the same time surgery is prerequisite for biopsy. If just chemo is being given then pet scan shows the most trustworthy data.


Not sure if the growth is rapid but the size is pretty big and can cause you difficulty to pass stool, you should visit the oncologist at the earliest. 

Posts: 247
Joined: Dec 2013

This is correct.  A CT is done to have a rough image of structures in the body for the PET scan to be superimposed on top of.  But the CT imaging done is not as detailed as a CT/contrast alone.  As to oncologists preferring PET scans and basing their diagnosis on this, this is the first I'm hearing this.  PET scans are an additional tool to use for diagnostics.  The usual sequence is to have a CT/contrast done first.  I rarely see PETs used a the primary diagnostic tool.  Again, PETs don't provide good quality imaging of structures in the body and they expose the body to huge amounts of radiation.  Radiaton exposure is cummulative.  I've had 4 PET scans over the 7 years of this wretched journey and I don't know how many CT/contrast scans as I've lost count.  This is why my oncologist is removing any further scans unless something pops up requiring it; despite my CEA still being abnormal.

ETA:  Forgot to mention another thing about why PET scans are not used as a primary diagnostic tool.  They're expensive and insurance companies are reluctant to pay for them even when used as a secondary diagnostic tool.  I've seen people report back on another support forum their insurance denying coverage for a PET stating the CT/contrast was good enough.

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