Podcast: PET Scans' Role in Cancer Diagnosis and Treatment


June 29, 2010 – Positron emission tomography (PET) has become an essential tool in detecting cancer throughout the body. PET scans allow doctors to detect abnormalities in glucose metabolism, which is a hallmark of cancer cells. In this edition of Breakthroughs in Cancer Research, radiologist Barry Siegel, MD, says PET scans help in the diagnosis and staging of cancer. Furthermore, he says PET scans can be used to determine if treatment has destroyed all of a patient’s cancer. Siegel says in the future, PET will play a crucial role in bringing about individualized cancer therapy, with doctors using one or more PET tracer to help determine if a particular type of cancer will respond to a particular therapy before therapy begins or soon after it’s started.

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On this edition of Breakthroughs in Cancer Research, we’ll talk about how PET scans help in the diagnosis of cancers throughout the body and the future of PET scans in cancer treatment.

Host: I'm Gwen Ericson, and I'm talking with Dr. Barry Siegel, a radiologist at the Siteman Cancer Center and chief of the division of nuclear medicine at Washington University School of Medicine. We'll be discussing why PET, or positron emission tomography, is so important in today's cancer medicine. Dr. Siegel, thank you for talking with me today.

Siegel: Glad to do so.

Host: Could you explain what PET is and how it works?

Siegel: Sure, PET is a nuclear medicine imaging technique that takes advantage of a particular group of radioactive tracers that allow us to really delve more deeply into physiologic processes in the body. Now, PET primarily looks at one specific tracer, which is a radioactively labeled form of the sugar glucose. The glucose is administered through a vein, we wait a period of time and then we use the PET scanner to take pictures that tell us about the use of glucose by different organs and tissues in the body. Now, with respect to cancer, one of the hallmarks of cancer cells is that they use more glucose than normal tissues, and consequently, the ability to detect this on an image is a way of detecting the presence of cancer, and it's a very effective way of doing that.

About a decade ago, a very important technological improvement occurred which is that the PET scanner device got put together with a conventional computed tomography, or CT scanner, and now almost everywhere in the United States we use devices that are called PET-CT scanners. And now we have the best of both worlds in one image set. We get CT pictures that tell us about the anatomy of a particular region, and we get the PET images that tell us about the glucose metabolism. We're able to look at these pictures separately or fused together so that we can get a very detailed overall picture of what's going on in terms of the distribution of tumor within a particular patient.

Host: Tell me more about the role of PET in cancer treatment today.

Siegel: So PET really has become very pervasive in helping to manage patients with many, many different kinds of cancer. This abnormality of glucose metabolism has been shown to be as, I said earlier, a hallmark of cancer cells, and with very few exceptions we see it with almost all types of cancer.

The starting place may be the use of PET to help diagnose cancer. The best example of that is a patient who may have a lung nodule discovered on either a chest X-ray or a CT scan done for some other reason. Yes, one way to find out whether that's cancer would be to biopsy it or remove it, but we've learned over the years that by doing a PET scan, we are very accurately able to tell whether that nodule can simply be followed because it's likely to be benign, or if it shows increased glucose uptake, it's very likely to be cancer and it should be removed.

In addition, for many tumors, once a cancer has been diagnosed we're now faced with the question of how should we treat that cancer? Do we remove it surgically, or because it has spread over a wider area, are we going to have to manage it with radiation therapy or if it's spread widely throughout the body are we going to have to just use chemotherapy or some other form of systemic treatment? A PET scan has been shown to be a very effective way of evaluating the total extent of tumor in the body when a tumor is first diagnosed. This is called staging of the cancer, and PET is very important in cancer staging.

In addition, after completion of treatment, after completion of a course of radiation therapy or completion of a course of chemotherapy, PET is used determine whether there's any residual tumor left, whether or not the treatment actually killed all of the tumor.

And then later on as patients who have been treated and seemingly are doing well, when things pop up either in their history or in the physical exam or on some other imaging study, PET is used to try to establish whether or not the cancer has recurred.

Host: What's on the horizon for using PET in cancer treatment?

Siegel: We're looking soon to start seeing combined PET and MRI scanners, magnetic resonance imaging scanners, and the real advantage of that will be that we'll be able to reduce the total radiation exposure from the exam if we can substitute the MRI, substitute the MRI instead of a CT scan.

There's more to characterizing the biology of tumors than just looking at their glucose metabolism. One of the strengths of PET that has been recognized for decades is that we can develop radioactive tracers of many, many different molecules that allow us to probe many different aspects of tumor physiology and biology. And there are many things coming down the pike and one will be mechanisms to look at tumor proliferation rates, how rapidly cells are growing, which many people believe will give more specific evidence about how a tumor is responding to treatment than we currently get from looking at glucose metabolism.

Another area of interest is assessing whether tumors are getting an adequate amount of oxygen, in other words whether there are areas in the tumor that have what's called hypoxia. Hypoxia makes the tumor more resistant to treatment, makes the tumor more aggressive. If we can identify the tumors that have hypoxia, then we can apply specific types of treatments to overcome the hypoxia and hopefully get better outcomes in those tumors. There are radioactive tracers coming that will allow us to assess the growth of new blood vessels in the tumors, so-called angiogenesis. There are many therapies now that are directed against angiogenesis. It's one of the most exciting areas of cancer therapy.

Another area of radiopharmaceutical development that we think is very interesting is using tracers that tell us about the presence of particular types of receptors in the tumor, and so there are therapies that are designed to either block the receptors or to stimulate the receptors that can be used to treat the tumor. For years we've been working on radioactively labeled estrogens as a way to tell us whether breast cancers express estrogen receptors, and the whole approach to hormonal therapy of breast cancer is driven by knowledge about the estrogen receptors. And over the years we've shown in a number of different ways that we can use imaging to predict whether a woman's breast cancer will respond to hormonal therapy by this type of PET imaging.

The best way to characterize what's coming in the future, because there's many, many things on the horizon, is getting us closer to this concept of individualized cancer therapy where we will use one or more PET tracers to help make key decisions about whether a patient's particular cancer is going to respond to a particular type of therapy either before we start the therapy or soon after we start the therapy, rather than waiting months to find out that we were on the wrong track and have to switch to another therapy at that point.

Host: You can learn more about the exciting new developments in PET imaing and other new approaches to cancer development and treatment online by visiting the Siteman Cancer Center's website at www.siteman.wustl.edu.