Radiology: PET/CT Scan
Guest: Dr. Leonie Gordon – Radiology, MUSC
Host: Dr. Linda Austin – Psychiatry, MUSC
Dr. Linda Austin: I’m Dr. Linda Austin. I’m interviewing Dr. Leonie Gordon, who is Professor of Radiology here at the Medical University of South Carolina. Dr. Gordon, one of your areas of specialization is in the use of PET/CT. Can you explain what that technology is?
Dr. Leonie Gordon: This new technology uses radioactive glucose to detect tumors and their spread. Then, because it’s a physiological imaging agent, we use CT to help us anatomically localize where these regions of abnormality are.
Dr. Linda Austin: So, then, basically, because it uses glucose; which is sugar, this is a marker of how metabolically active a given area of tissue is? Is that right?
Dr. Leonie Gordon: Yes. Usually, tumors are metabolically more active than the surrounding cells, especially if the tumors are growing very rapidly. The nice thing about using radioactive glucose is that there are no adverse reactions, because it’s sugar water, and we use it in our bodies all the time. So, we don’t ever have to worry about someone having an allergic reaction to radioactive glucose. And we can look at the whole body at one time, so it can give you an idea of where a tumor is and where it has spread to.
Dr. Linda Austin: Now, as a psychiatrist, I’m used to seeing, at least in journal articles, photographs of PET scans of the brain, where different areas of the brain light up in different colors, depending on how active they are. Tell us about how these PET CTs can be used in the brain. What sorts of diseases can you diagnose?
Dr. Leonie Gordon: Within the brain, it’s particularly useful for looking for dementias, including Alzheimer’s disease. People with Alzheimer’s disease and dementias in the area of the brain that’s affected may not metabolize glucose as well as in the other, normal, areas of the brain, so we can see an area of decreased activity, or lighter activity. That would then tell us that area has been involved in the disease process. We can also see strokes. And sometimes we can see metastases or spread in the brain. But, you know, your brain uses radioactive glucose as food for thought, and for you to function. So, occasionally, it’s a little hard for us to tell normal brain from abnormal brain in some of the metastases of the tumor.
Dr. Linda Austin: How advanced does Alzheimer’s have to be before you can start detecting changes?
Dr. Leonie Gordon: The patients have to have been on therapy that has not really been effective for at least six months, and then we really can see abnormalities. Occasionally, we have seen abnormalities before this time. And the government will pay for a PET scan of the brain after about six months of failed therapy.
Dr. Linda Austin: Let’s go on to other areas of the body. What is the most common anatomical site that you look at when you do a PET/CT?
Dr. Leonie Gordon: It depends on the tumor type. So, if a patient has a melanoma, and they suspect spread of the melanoma, we will then scan the patient from the top of the head, down to the toes. However, in many of the tumor entities, we’ll just scan from the eyes, down to the thighs. So, we can look at your bones in those areas. We can look at all your lymph nodes in those areas. We can look at soft tissues that are involved. We can look at your liver. We can look at your spleen. So, we can really see a lot of areas of increased activity in one scan. And patients really like to know everything that’s going on with them from just one study, rather than having to go from study to study to try and work out exactly what’s wrong with them.
Dr. Linda Austin: So, this is used, then, in newly diagnosed cancer, as well as, I guess, to track whether there’s been a recurrence of the tumor?
Dr. Leonie Gordon: In many cases, we don’t actually look at newly diagnosed cancers. For example, in breast cancer, mammography is much more effective than PET/CT. But once you’ve had the initial staging of the procedure and you want to know if it has spread, or the doctor suspects that it has spread, this is a good test for you to undergo, because, again, it will show all the areas that are involved.
Dr. Linda Austin: Tell us about the procedure, of actually having this done.
Dr. Leonie Gordon: We specially order the radioactive glucose for you. You come into our department and we check your blood sugar. We want to make sure that you’re not diabetic or that your own blood sugar isn’t too elevated. Many times, we’re looking, if you’ve had some therapy, to see how effective your therapy has been. So, it’s very important that we have the right blood sugar level and the right uptake of our radioactive glucose in various abnormal sites.
So, we inject you with the radioactive glucose, and then we ask you to lie quietly, in a quiet room, for approximately 45 minutes to an hour. And the reason we ask you to do that is we want the glucose to go to your tumor cells. As you know, radioactive glucose also goes to muscles. So, if you chew gum; if you talk; if you whistle; if you sing to yourself, all of those muscles, then, will take up the radioactive glucose, and the quality of the study will be decreased. So, we try to put you in a quiet, warm room for about an hour, and then we scan you. The actually scanning takes about 13 minutes, and that will be the PET portion and the CT portion.
Dr. Linda Austin: So, it sounds, really, as if it’s a fairly simple, benign procedure.
Dr. Leonie Gordon: It is a simple procedure. It only takes a little longer than it would for just a CT, because we have to wait for the radioactive glucose to get taken up by the cells; the abnormal cells, that we want. So, if you come in for a PET/CT, it will probably take about two hours from beginning to end.
Dr. Linda Austin: How can you use PET scans to track treatment response?
Dr. Leonie Gordon: We have a way of quantifying how much glucose is taken up by your tumor. If you’ve been on therapy and we’ve noticed that there’s been more than a 50 percent decrease in the uptake of the radioactive glucose in your tumor or its spread, we then know that your therapy is effective. If there has not been that amount of change, the doctor, then, will change the therapy, because he knows it hasn’t been working.
Dr. Linda Austin: Dr. Gordon, thanks so much for talking with us today.
Dr. Leonie Gordon: Thank you very much.
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