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Medical Intel

Sep 25, 2018

PET-CT scans provide detailed information on where cancer is located, whether it’s spreading and if treatments are working. Dr. Carlos Garcia explains how this test works, what to expect if you’re having one and how your doctor uses the results.



Intro: MedStar Washington Hospital Center presents Medical Intel where our healthcare team shares health and wellness insights and gives you the inside story on advances in medicine.

Host: Thanks for joining us today. We’re talking to Dr. Carlos Garcia, Medical Director of Nuclear Medicine at MedStar Washington Hospital Center. Today, we’re talking about the benefits of PET CT scanner. Dr. Garcia, what is a PET CT scanner and when is it used?

Dr. Garcia: So, a PET CT scanner is kind of a gold standard for cancer imaging. Just like in the past it was called a CAT scan, now they’re called CT scans, are for anatomic imaging of multiple causes, whether they’re cancer or not. PET CT is 99.9 percent dedicated to cancer imaging. The difference between a PET scanner and a CT scanner is that a PET scanner will rely on the cells being alive and consuming a specific type of substance that makes them show up on the scan. This substance is normally either produced or it circulates in your body. And we add radiation to it and that’s why we can see it on the images. So, it’s a combination now of anatomic imaging from the CT portion of the exam and functional or metabolic imaging from the PET portion of the exam. PET stands for positron emission tomography. Positrons are just basically an energy source that will, you know, produce an amount of radiation that we can translate into images, and the more active a cell is--especially the cancer cells tend to be more active than normal cells--the more they’re going to emit this type of energy, the more they’re going to take up this type of radioactive substance that they would normally not, and that’s how they show up in a more avid, or they light up on the scan, if you will.         

Host: So, how is the substance introduced to this other substance that you mentioned in order for it to show up on the PET scan?

Dr. Garcia: So, what we do is we make the patient the source of the radiation. We inject the patient with a modified version of glucose, which is the most common one; that’s why I am going to use that as an example. It’s called fludeoxyglucose, or FDG for short. You’ll see that many of our exams have acronyms for that same reason. It’s kind of a hard, long word. So, we inject the patient with this glucose substance, and all the cells in the body normally will use glucose as energy. Cells that replicate faster or grow out of control, as cancer cells do, will use more glucose because they require more energy to sustain this growth pattern. So, the cells that take up more glucose will take up more of the radioactive glucose that we have injected into the patient, and that’s why they will shine in comparison to the background of normal cells. These cells will look bigger, darker and brighter because they just take up more of the radioactive substance that we tricked the body into taking because it doesn’t know it’s radioactive; it just thinks it’s glucose.  

Host: So, what are the advantages of combining a PET scan with a CT scan for cancer?

Dr. Garcia: For many, many years, when we did not have PET images available, we only used CT imaging, which could provide us only with an anatomic version of whatever is going on inside the body. The problem with this is sometimes you can see a tumor—we’ll just use that as an example, and only the inner portion of the tumor might be where all the cancer is, and the rest of it might be just inflammation resulting from the presence of that cancer or tumor. The PET scan can differentiate that sometimes. It can show you what is the actual size of the live tumor inside of a structure that might be, let’s say for example, 5 cm larger, but it’s not all tumor. This helps in many ways to guide the therapy. One of the main applications of knowing this difference between what is functional or what is metabolically alive versus just the anatomy, is that when you apply this to radiation purposes for treatment, for example, the radiation field that will be attached, I’m sorry, that will be used for that particular tumor will only be the size of the part of the tumor, or the part of the mass, that is actually alive, so you can actually make it a little more circumscribed and more directed. So that’s one of the applications when you talk about it. Now it’s called all hybrid imaging--that is the gold standard nowadays of all cancer or oncologic imaging. And now the new hybrid imaging scanners like the ones that we have here, will overlap these images, and they’re called fused activity, and you can definitely see the background and over the background of anatomy overlap with the cells that are actively replicating that turn out to be cancer cells.     

Host: So, having the fused images really provides a deeper layer and a deeper perspective for both the imaging team and the physician. Does the patient also get the chance to see those images?

Dr. Garcia: When we have the opportunity to show the images to the patients, by all means. Nine out of 10 times, the physicians that have ordered the tests are comfortable with the patients knowing the results. We always want to extend the courtesy of the referring physician to be directly involved with the patient and them communicate the results, but we have had scenarios in which the patients are very comfortable knowing and they are very, very well versed in their own disease process, as it should be nowadays, and we are happy to show them the images. The overlapping of the anatomic portion of it and the live cell, the metabolic portion of it, it completely takes away from having to even point at the screen. Everything becomes very, very obvious and you can use different color schemes to bring out certain cell types, so it makes a picture worth a million words instead of a thousand words. And all the physicians within the hospital, they have the opportunity to be able to see these images on Enterprise-wide imaging viewer that they have access to as well. So, that makes our job very easy. Even though they’re always welcome to come to the reading room and have us show them the images directly, we can have phone conversations, them looking at the exact same images that we are looking at, and we can tell them slice number and position, and target everything they need to know.

Host: Are there certain cancer types for which PET CT scan is most applicable or certain body parts that are easier to do?

Dr. Garcia: That’s an excellent question because, for a long time, PET CT imaging was considered to be the, you know, savior for all types of cancer, and that unfortunately is not true. It has limitations and then it has indications that make it much more favorable. So, off the top of my head, I can tell you the top three indications that we use it here at the hospital are for breast cancer, lung cancer and lymphoma.  So, these tend to be tumors that are very metabolically active. When I say metabolically active, it means that they have a tendency to take up more glucose than normal cells would, than other types of cancer would, so as long as they take up this increased amount of glucose, they will be brighter on the images and they will really stand out from the background, making the ability to detect them much, much easier.

Host: It really makes the cancer sound like a living thing and like a living disease, if you will, as opposed to just this abstract.

Dr. Garcia: I’m going to actually start using that way of explaining it because that actually sounds exactly right. That is, it’s a live being that has a mind of its own sometimes, and our job is to be able to detect it early, be able to apply what we see to a treatment program, and then, after the treatment program, be able to monitor the response to the therapy by seeing whether that activity that translates into a lot of glucose uptake, seeing if it went down, meaning the number of cells is going down based on the therapy. If you have less amount of cells, it’ll be less glucose that will be taken up and the shine on the exam, if you will, will decrease over time.  So, we measure, we have units to measure the intensity of this activity and we use them very specifically, you know, to monitor the response to therapy because those units should go down as the number of cells within the cancer start dying as a result of the therapy.        

Host: If my doctor tells me that I would need a PET CT scan, what should I expect from that appointment? What will that process look like?

Dr. Garcia: So, the first thing that you’re going to do is you’ll be contacted by one of our staff members from the PET CT Center, and they’re going to ask you to prepare your body to be able to absorb the glucose better, and that’s going to require for you to be fasting for at least six hours prior to the examination. So, the first glucose that you will receive, meaning the first source of energy that your body that has now been without food or drink for six hours, it will be starving. So, you prepare the body to eat something, and then the first thing that it sees is the glucose. And like I said before, it doesn’t know that it’s radioactive. It just thinks it’s glucose, so it’ll latch onto it immediately, and that way you won’t have any competition with glucose from your diet, I mean, which are carbs basically from your diet, competing for a space to latch onto certain cells. So, everything that you will have will be radioactive glucose it’ll latch on. And you arrive to the center. We give you the injection. It’s going to an intravenous injection in your arm. You’ll sit in a quiet room for anywhere between 40 or 60 minutes, and the reason why you want the quiet room is because you don’t want any stimulus to any organ in your body, so we’ll get you in a nice warm-temperature room, you’ll relax, and then about after 60 minutes from that, we will place you in the PET CT camera, and with the new cameras, the amount of time that you will spend under the camera is a lot less, so you’re looking at anywhere between a 20 to 30-minute scan from the level of your eyes down to your mid thighs. And once you conclude that portion of the exam, then the images get sent over to the radiology reading room, to the nuclear medicine reading room, for interpretation.   

Host: How does that doctor then use the imaging to suggest treatment for me?

Dr. Garcia: So, we use a staging system, and I’m going to just use cancer, you know, as an example, which is the majority of the reasons why you do a PET CT scan. There’s a staging system to know how far or how advanced, your cancer is. And depending on how advanced your cancer is, the treatment modalities will change. Let’s use, for example, if we have a patient with lung cancer and they have a small lesion in a very circumscribed area and nothing anywhere else because the PET scan did not show that there was spread to any other organ in the body, then, in these particular cases, one of the treatment modalities might be surgery, for example. You know, I’m not a surgeon, but this is, you know like, this is, you know, one of the treatment modalities, one of the treatment options would be surgery. If, for example, the same patient has that same spot in the lung, but also has spots in his liver, also has spots in his bones, surgery may no longer be an option and a more systemic approach is necessary, meaning something, a form of therapy that will apply to your entire body since there has been spread.  So, it can guide the clinician to knowing what the treatment options are for the patient, and that opens the discussion, you know, with the patient that these are now your options and this is your staging, you know, this is what we consider it to be. Because nowadays patients will do a lot of research on their own, and they come in asking you, you know, like, what is my stage? Am I stage I or stage II? What are my options based on these stages? So, that’s really, really what helps to kind of tell the patient where they are and what their options are.  

Host: So, really mapping the progress of that tumor through the body and then the patient’s progress, thereafter, is mapped by the images.

Dr. Garcia: Correct. Mapping is a good word to use specifically for this because you will do a PET scan in various clinical phases. One of them is going to be for initial diagnosis. If a patient comes in and has an x-ray and they see a small spot on his lung, that could qualify him for having a PET scan, and they’ll isolate a nodule. Then, after that, they will apply whatever treatment option is available to the patient based on the images, and then you will have another PET scan after the therapy has been installed to monitor treatment response. So, there’s an initial treatment strategy and then a subsequent treatment strategy. That’s how PET scan is divided nowadays.  So, it’s early, it’s a very important early on in the initial staging and diagnosis, and also in the monitoring treatment response phase to see if it all cleared up, if it’s spread more, or if it’s actually regressed completely. And we see patients that sometimes showed up in their doctor’s office with a small tickle in their throat. And it turned out, then they went over to their ear, nose and throat doctor, and they saw a little growth--you know, a little something, a little bulging inside their throat. And when they ordered a PET scan for this, it turned out that it was not just in that little spot, but it was in many other areas within their neck or within their chest. And, like I said, you know, once you know that, it’ll change the treatment options, but I can say definitely, this is something that I share with my colleagues, is that you sometimes, you know, you get, your day brightens up, when you compare it to a study before that showed a tremendous amount of disease, and after chemotherapy or radiation, it’s all gone. So, it’s always nice to have that dramatic effect when you see live cancer cells everywhere and all these cells are dormant--you know, they disappeared basically. We call them night and day scans.    

Host: For many imaging tests, patients have asked questions about or have been concerned about the level of radiation to which they’re exposed. How does the PET CT radiation dose compare to MRI or another imaging?  

Dr. Garcia: That’s an excellent question because that is the one thing that people will worry about a lot is radiation exposure. Yes, PET scanning alone will produce much higher radiation exposure than a chest X-ray, but you have to think about the amount of information that comes as a result of that, you know, slightly over, you know, the normal degree of exposure. People sometimes don’t understand that you just by standing around, you know, are getting a little bit of radiation from nature. And living in Denver for a year, you actually get more radiation than by having a PET scan. So, it all depends, you know, like on what kind of information you get out of it. When you do CT imaging of certain parts of your body, depending on what part of your body you’re going to image, that part of your body gets an amount of radiation. PET scanning is the injected dose that will distribute throughout your body, so the dose that you receive will be spread out through your entire body, and that is, in essence, less of a dose to each organ in your body than if you only imaged one particular area at a time. So, it is a little bit more than CT alone if you only did CT imaging, but again the risk, benefit, and the amount of information you get for only a small amount of extra radiation, which is very, very below what the maximum amount of radiation you can receive in a year is, you know, is a wealth of knowledge.  

Host: Are there any patients for whom PET CT scan just is not an option because of either the radiation dose or another complication?

Dr. Garcia: Actually, I can’t think off the top of my head of a case in which you could not use a PET scan. We more, we more will see it in cancers that either don’t take up glucose, radioactive glucose, so it’s not the appropriate test for that particular type of cancer. Other limitations, of course, are that it has been proven through many, many studies that it is not the best examination for that particular type of cancer. There are some types of diseases that we wish we could use them on there, for those, but it’s not approved to be used for those cases.

Host: Thank you so much for joining us today.

Dr. Garcia: Oh, it’s my pleasure. Thank you for having me.

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