PET scans

HeatherBLocklear

Hi all my fellow Elephants (capital e on purpose) --

Can anyone explain to me exactly what a PET scan is, how it works, and what it can, and cannot, detect? Can it, for example, pick up stray cancer cells floating about in the organism, or must they have settled somewhere and started to colonize? Is there such a thing as a false negative with a PET scan? In other words, can one have cancer somehwere that's not detected by the scan? How sensitive is it compared to, say, an MRI? A regular CT scan? Or is there no difference?

As you can see, I'm a scientific moron. That's why I became a PhD instead of an MD which would have been my first choice. I guess the medical profession can be glad it was spared my membership (it would be better off without a number of other persons of whom I'm thinking with great resentment, but whose names I'll be kind enough not to mention in public). Sorry for the digression.

Thanks for any information the scientific minds (Otter, Tender, the various Ann[e]s) can offer.

Hugs to all,

Annie

shari1232

http://www.radiologyinfo.org/en/info.cfm?pg=PET&bhcp=1  I have had this procedure.  If you have further questions, give a shout.

HeatherBLocklear

Hi Shari,

Thanks so much for the link. I've already had the PET scan, so know how the procedure goes. I guess a number of my questions above remain, if anyone has the answers?

Thanks again.

Annie

anneshirley

My understanding from previous research, before my PET scan, is that the cancer has to be a certain size before it can be detected by a PET scan (don't remember the size but not that small as I remember). If I can find my research I'll post the size.  And yes, there can be false negatives and positives.  Negatives, I assume, if the cancer is not yet sufficiently large in size.  I had a false positive, for a long-standing cyst on my back.  My oncologist discounted the radiology report because the area that had uptake was not a usual place for BC to migrate, and I confirmed it by identifying it as my long-standing cyst. Don't have answer to the other questions, although I did know them some time ago.  Again, I'll read through my research.

Maryiz

Annie, from my understanding, the PET does not pick up anything under 1 cm, so if you have anything tiny, I don't believe it will show up.  Yes, there can be false positives also.  I have mixed protocols as far as not exercising beforehand and low carb diet to get an accurate reading, but none of this was told to me before my test, so I don't know how reliable that is.  Hope this helps. Maryiz

Pat634

From what I understand the Pet scan will "light up" any rapidly dividing cells. It not only showed my breast cancer and its spread to my lympth nodes but also adenomas and polyps I had and both of those are supposedly non cancerous.

TenderIsOurMight

 

Hi Annie,

 

As usual, you cut right to the bone with your insightful (not inciteful, mind you) questions. I don't know enough to answer them. I'm going to give a link to an abstract title which shows some of the complicated discussion surrounding PET scanning, and maybe it will help a little.

 

Definitely though you can have false negative results with PET. No single imaging test has 100% ability to show exactly what is present as yet, regrettably. I do understand that small breast lesions (< 10 mm) are indeed more subject to false negatives on PET. This is reviewed in the article (nodular vs diffuse, degree of tumor differentiation...).

 

Others will be by to help. I know it's a tough time, and I'm sorry. I hope your doctors get specific in plan real soon.

 

All my best to you and yours,

Tender

 

¹⁸F-FDG PET in Detecting Primary Breast Cancer

http://jnm.snmjournals.org/cgi/content/full/48/10/1751 

 

otter

Annie, I'm marking my place.  (See various discussions on the Bitch thread to find out more about usefulness of place-marking.)   I've been traveling for the past 48 hours and was completely webless until about half an hour ago.

I'll take a look at the reading assignment Tender gave you, and I'll see what else I can find....but I won't be able to get back to you about it until tomorrow afternoon.

In the meantime, here are some extra hugs:

{{{{{{{{{{Annie Camel Tail}}}}}}}}}}

otter (can you feel us all leaning against you?)

otter

OK, Annie, I'm ready to submit the results of my homework assignment.  First, I read the article Tender gave us.  The authors made this statement:

"In our study on 111 patients with suspected breast cancer, 18F-FDG PET alone had a sensitivity of 48%, a specificity of 97%, a positive predictive value of 98%, a negative predictive value of 40%, and an accuracy of 61%.  In that study, we found a sensitivity of only 23% (7/30) for primary breast cancer lesions that were 10 mm or smaller."

My translation of that article:

1) PET scans do not work well, and therefore are rarely used, to detect or assess primary BC lesions (i.e., for initial screening and/or dx);

2) PET scanning alone (a PET scan that is not done concurrently with another imaging method) is not very sensitive.  A PET scan cannot detect small tumors--even those that might be detected with other techniques, like CT, MRI, or mammography.

3) Because of its rather poor sensitivity, there are "false-negative" PET results; and the "predictive value" of a negative scan may low.  OTOH, "false-positive" PET results are rare, so a positive PET scan is a pretty good indication that something is going on. 

I know that's not what you wanted to hear at this point.  We would all like an imaging method that could detect individual cancer cells as the little bastards break free and launch a new invasion...but no such method is available for human bodies--at least, not yet.

I did find some information about reasons for the limitations in the resolution with PET scans.  First, I need to quote some stuff about the principles, so I can explain the limitations (I know you've probably seen this sort of thing already):

"[PET = positron emission tomography] ... uses the radioactive isotope Fluorine-18 attached to a glucose molecule. The combination is called 18-F-FDG, which stands for 18-Fluorine-fluorodeoxyglucose. The molecule is taken up by cells as glucose, so it localizes most strongly in cells such as tumors and the brain [and the heart] that are metabolically active. The F-18 decays by emitting a positron, which can travel up to 2 mm before bumping into an electron and emitting two 0.511 Mev photons.

[PET produces images] by taking advantage of the fact that when the positron and electron annihilate, the two photons produced travel away from each other at 180 degrees. The detector is in the shape of a donut with the patient in the center of the donut hole. Since each F-18 decay produces two photons, since the photons travel 180 degrees away from each other, and since the two photons from a decay strike the detector array virtually simultaneously, the detector: 1) assumes that any two photons hitting within 30 nanoseconds of each other come from a single event ..., 2) draws a line between the two photon "hits" in order to locate the site of emission somewhere along that line, and 3) repeats the procedure for a huge number of decays. The points where the greatest number of lines intersect are the areas of greatest cellular activity."  (http://www.madsci.org/posts/archives/2001-03/984008649.Me.r.html)

Okay--is everybody still with me?  Let's set aside the unavoidable image of an F-18 zooming around within our bodies, parking inside a tumor, and exploding to emit positrons that bounce off the walls of the tumor.

Here's the problem: 

"The spatial resolution of PET images is limited. This is explained by many factors, both physical and patient-related. The positron pathlength after emission causes a blur of 1 to several millimeters, depending on the selected radionuclide. The two photons often do not have an exact 180 degree angle because the positronium may have a little residual speed. The photons may deviate slightly inside the body due to scatter before reaching the detectors. The detectors have a finite resolution. The reconstruction algorithms yield an estimation of the true distribution of molecules. Furthermore, the patient may move during the image acquisition, for example due to breathing.  And last but not least, the amount of detectable positrons is limited on purpose because of radiation burden considerations.  All together, it can be considered impressive that the spatial resolution of current detector systems is in the range of 5 mm in all directions..." (http://knol.google.com/k/wouter-vogel/pet-scan/3bjk7wg4q9a0z/5#)

My report is getting very long and I'm afraid you've already fallen asleep...but I need to note that I read several other articles painting a better picture of FDG-PET scans than the one Tender gave us.  One article said this:

"In breast cancer, FDG PET has been used for defining the extent of recurrence or metastatic disease.  FDG PET modality of imaging has 88% sensitivity and 80% specificity for primary breast lesion, 61% sensitivity and 80% specificity for axillary metastases, and 93% sensitivity and 79% specificity for metastatic disease.  ...  The sensitivity to detect malignant lesions decreases with low grade tumors and lesions <1 cm..." (http://www.hemonctoday.com/article.aspx?rid=30193)

One of the newer strategies to increase accuracy and usefulness of PET scans is to combine the PET with CT--basically, to do them in the same procedure.  It's called "FDG-PET/CT".  The CT scan provides the anatomic detail that is missing with a PET scan, and somehow approves the apparent resolution.  When the digital image of the CT scan is superimposed over the digital image of the PET scan (a feat that is accomplished with computer wizardry), the suspicious areas in the PET scan can be identified.  I don't know how sophisticated a cancer center needs to be, to do a PET/CT combo.

My head is spinning at this point.  Time for 2nd breakfast...or is it 1st lunch already?

Hugs, Annie...

otter 

Anonymous

I believe the size of a pea (about 7 mm) is the smallest tumor that a PET scan can typically detect. It certainly can't detect individual cells. There are some experimental instruments that claim to be able to detect free circulating cancer cells, but I don't believe that any of these are approved for in vitro (or in vivo) diagnostic use in the US.

They do not detect rapidly dividing cells. The injection they give you tags a radioactive tracer onto glucose molecules. Cells that exhibit increased metabolic activity take up more glucose (tumor cells typically show increased metabolic activity). So tumors, a collection of billions of hungry cells, light up on the scan.

Any tumor less than the detection threshold (7mm) will not be seen. I don't know if you can technically call it a false negative, since the scans are not claimed to be able to detect tumors that small. A false negative would be an assay that claims to be able to detect something, but fails to do so. Some areas of the body uptake the glucose faster than normal cells, but most radiologists are aware of these and make appropriate notations in the report.

Did you not receive and discuss your report with your oncologist? I grilled mine for about 20 minutes on the results.

HeatherBLocklear

Thanks to all. Again, you're incredible! I have an appointment with the oncologist at 8:00 in the morning and will grill him at that time. So far, have only spoken to him over the phone when he assured me that false negatives with a PET scan "don't exist." So God only knows. I live in a community where it's more or less taken for granted that women don't want to hear negative stuff, so maybe that's the reason for the half-truths. I really don't know. I'll keep you all updated, but I'm really scared. I guess most of us are, on and off. It does wear a body out, though, doesn't it!

Love,

Annie

lmb1104

With regards to Pet Scans...Did most of the people that have had these scans have them at the time of original diagnosis or was this performed following treatments to assess possible reoccurances?  Thanks.

Lisa

vhqh

Mine was done shortly after diagnosis, I had a CT and bone scan done, CT showed liver lesions so the PET was ordered to check on those, liver turned out benign but a femur met showed up that the bone scan missed.

hopefullady

I had already had lumpectomy and radiation.  Then I had a MUGA and a PET scan before chemotherapy.

Chris 

CanItBeTrueNH

New England PET Scan -Here is the web site. ( the picture is from the web site )

http://www.nepetimaging.com/pt_info.html

http://www.nepetimaging.com/casemonth_august_05.html  ( Breast Cancer info on web site) .

I had a PET SCAN and it helped by showing the size of my tumor and node involvement.  This test is around $3k -$6k depending on your insurance coverage and if you add a bone scan with it.  

Check with your insurance to make sure you are covered. 

Feel better everyone.

CanItBeTrueNH

Today during chemo, I watched a  frail woman in a wheel chair along with her husband heading to a chemo room next to us.  She has a very serious stage  of ovarian cancer.  Her husband later stepped outside of her room and sat crying uncontrollably . She was not doing very well.

While walking in the hallway, I talked to one of the staff members passing out drinks and sandwiches and she shared with me that ovarian cancer is extremely hard to detect and is usually caught a very late stage.    At that moment we felt very sad for her and her family.

It made me think of my PET scan and how much comfort I feel to have had one.  If I had any cancer anywhere else in my body over 1cm it would have shown it on the test.

My prayers are going out to the lady and her family tonight.

Feel better everyone.

susan13

I was just thinking of this the other day.  Even though going thru this disease I feel kinda blessed that it is not more serious then others.  I feel blessed for all the good I have in my life.  So many people young and old are suffereing so terribly with this disease. When I hear of or see children with cancer my heart just aches.  Amazing how this dx can change your life.

CanItBeTrueNH, I hope you feel ok during the next few days. Keep strong. Everyone!

Sue

ibcspouse

Just to put in my two cents on Pet scans, I will give you my wife's experience at major cancer center in TX.  They do intial staging with all the scans(brain  mri, breast mri, ct, us, mamm, muga bone and PET/CT.  As well as a CTC count.(individual circulating tumor cells in blood).  Once treatment begins, in wife's case PET/CT scans and CTC counts are done after every third treatment.  In wife's case Onc was aggressive looking for complete cinical response before surgury, since she was stage IV, this was tuff on her due to SE's with out break for 9 months.  The Dr would change Types of Chemo as soon as he saw a slowing of response to Chemo on PET/CT, CTC counts or CA 27.29.  So TAC plus Avastin, Gemzar and Carpo plus Avastin, Ixempra plus Xeloda, Ixempra only all in past 9 months, but, her PET/CT on the 20th showed no apparant breast cancer, all lymph nodes that did lite up are now clear.  Ovary tumur gone, or at least below detection.  CTC count zero, and CA markers at high normal. 

Long story to say that use of PET in staging and having comparative imaging during treatment seem to be a good protocol.  Interesting, notation Cemo brain can be detected on PET as decreased of FDG uptake in cerebrium