Home Forums Melanoma Diagnosis: Stage IV Brain Met Determination-PET or MRI?

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    Since the beginning of my treatment for Stage IV, I have been faced with suspected, real, and treated brain mets. I thought for a long time that the MRIs I had were the sole judges of whether they existed, where, and how large. Someone told me that the PET scan does not scope the brain, as it could cause radiation damage to that sensitive organ. In reading my latest PET scan report, I found: “No evidence of abnormal hyperetabolic lesions within the brain.” The brain MRI indicates a spot once held by a definite brain met has expanded a bit and changed in shape since SRS in December. Should I be heartened by the PET report in deciding whether this expansion and change is a manifestion of ipi and SRS rather than a growth in the tumor itself? Or was I right originally in thinking that the PET really can’t tell you much (I had earlier thought “nothing”) about what is going on in the brain?


    From what we’ve been told, and from our experience, a PETscan doesn’t tell you anything about the brain…a MRI is the best test. The PET only shows you areas that are “most likely” active disease on the body.


    Hey Frank.

    As I understand PET measures relative biological/metobolic activity by how rapidly the tissue picks up the radioactive sugar and cancer typically picks up a lot relative to the surrounding tissue. Trouble is the brain is an organ with naturally high metabolic activity so that it too readily picks up the radioactive sugar. This is why the entire brain (as well as some other organs) glows a bit when you see the PET image. This background glow combined with PETs ~6mm resolution floor and maybe lower biological activity of some tumors (because they’re slower growing and therefore have low sugar uptake) can make it a bit difficult to see an active tumor against the background glow of the brain. I liken it to trying to see stars while in a light polluted city. The background glow hides all but the largest, brightest stars. You have to get out into the country (the abdomen and lungs for instance) where the background glow is low to see the smaller, dimmer stars.

    That being said, if the tumor is active enough and big enough, it will stand out against the background glow of the brain on the PET image. Your’s does not. If you can’t see that “star” on the PET, and the shape and other visual impressions in the MRI suggest edema and/or a bit of surrounding tissue necrosis, then that might indeed be good news but it would have been nice to have a PET taken right before the start of radiation for comparisons.

    Hope this helps.



    All my PETs have been from the crown of my head to my toes, therefore including the brain. My first brain tumor *was* first spotted on a PET scan, as a large glowing sphere. Of course then they did an MRI to confirm. It was already a fair size at that point (bigger than the 6mm floor Jeff mentions).

    But I really have no idea if they do or do not ever consult the PET (if done at the same time as the MRI) when analyzing ambiguous areas of a brain MRI.

    I do know that my oncologist would still like something even better than MRIs, particularly (as in your case) to distinguish edema/radiation effect from actual tumor activity. Those interpretations in my case seems to be a judgement call by the radiologist and radiation oncologist, based on location, type of changes, etc. E.g., I’ve seen “an area of the brain prone to radiation effect” show up on my radiology reports, where there was growth that was interpreted, and apparently was either radiation effect or IPI changes, for the first couple months after Gamma Knife and IPI. Those areas have been stable or shrinking since then though.

    PS Hope this post is on firmer ground than my previous one about IPI skin rash (which was more IL-2 than IPI).

    – Kyle


    Oh, and regarding “…my oncologist would still like something even better than MRIs,” I also know that the PET scan is not the tool he wishes he had.


    Thank you, Jeff and Kyle. I was once a school principal, and I would have hired both of you as teachers. You are outstanding!!


    BuffCody, I found out a little more about how PET/CT vs. MRI from a webinar at the ABTA (American Brain Tumor Association), “What Patients Need to Know About Radiation and Nuclear Medicine”, http://www.abta.org/understanding-brain-tumors/anytime-learning/” class=”bbcode_url”>http://www.abta.org/understanding-brain-tumors/anytime-learning/ What I heard is making I should try to get my PET and MRI scans synched up again.

    The discussion begins just after the 50 minute mark. A listener asks, “Pros and Cons of MRI vs. PET in brain tumors”. Paraphrasing… all transcription mistakes are mine.


    MRI gives excellent image of the brain anatomy. Can see grey matter, white matter, different parts of the brain, etc. It doesn’t do as well what is functionally going on in a tumor or remnants of tumor. One application of PET is in tissue changes after radiation therapy. In MRI you can tell that something has gone on, but very hard to tell if it is recurrent tumor, or radiation necrosis. Sometimes on MRI they look exactly the same. With the PET, if there’s uptake, meaning metabolism of sugar, in the part of the tumor we think might be necrosis, that means it’s not dead, so could be recurrent tumor.

    A new kind of scanner (about 10 installed in the U.S.) is the MR/PET scanner. One is at Mass General (MGH). To know as early as possible if there is a response to therapy, a combined MR/PET scan sometimes can help. Combining MRI and PET gives most complete picture of what may be happening in brain. Try to look at both at the same time, the information is complementary.


    There’s something on the MGH web site about there MRI/PET scanner here, http://www.massgeneral.org/cancer/about/newsarticle.aspx?id=1560” class=”bbcode_url”>http://www.massgeneral.org/cancer/about/newsarticle.aspx?id=1560

    – Kyle



    Thank you for that explanation, really clear. When I had my PET scan for the tumour I now see the reason for the brain tumour not being picked up. It was January, so possible it was not one of the brightest stars, lung, abodmen, pancreas, all being far easier to spot.

    Also MRI was used to confirm the brain tumour and in fact was used just before ippy drug.

    The more ‘we’ understand the better chance we have.

    Thanks again – XX :)



    Thanks for the reference to the Webinar, Kyle. It helped me understand the potential risks involved with the various uses of nuclear medicine, which the lecturer concludes are almost always worth it in addition to the specific question of the relationship of the PET scan and CT scan and MRI. It will lead me to ask about the possibility of using one of those multi-ability PET-MRI machines if the recommendation is to proceed to craniotomy without greater certainty that what is being dealt with is a melanoma met rather than a necrosis.


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