Puts Form and Function on the Same Page
Robert S. Hellman, MD
Medical College of Wisconsin Radiologist
To diagnose and treat cancer, physicians rely on anatomical and functional images of the body. Dr. Robert Hellman, a specialist in nuclear medicine at Froedtert & Medical College of Wisconsin, says the recent trend toward "fusing" the two kinds of imaging has helped doctors identify and locate cancers more accurately.
Q. How does nuclear medicine imaging differ from other imaging approaches?
There are two areas in cancer imaging. You have anatomical imaging, which constitutes the larger part of cancer care, and you have functional imaging. Nuclear medicine uses radioactive agents to look for specific functional changes that help us discriminate "bad" tissues from "good" tissues.
For example, we use a radioactive sugar called fluorine-18 labeled glucose. Typically, a tumor – which is often growing at a faster rate – uses sugar at a faster rate. We use that indicator to identify the tumor.
In the case of lymphoma, we use a radioactive tracer called gallium. Radioactive gallium tends to accumulate in active lymphoma. As the tumor activity diminishes, we also see less gallium in the lesion. In essence, PET imaging detects the heightened metabolic activity typical of cancer cells.
Q. Where does PET fit in?
Positron emission tomography (PET) is a 25-year-old technology, but functional imaging using PET radiopharmaceuticals has really taken off since the early 1990s. Since that time, moderately strong evidence-based data showed that PET functional imaging made a real difference in how we treat patients for multiple tumors – for example, lung cancer, lymphoma, colon cancer, esophageal cancer, breast cancer, melanoma, head and neck tumors. The data showed PET probably results in a change in patient management in about one in three cases.
Q. How does PET imaging change patient management?
Well, for example, if you're looking at lung cancer and you have a one-centimeter-size lymph node, that could either be normal or it could contain cancer cells. You can't simply look at the size in this case and decide whether or not this is cancer. With our PET scanner, we're able to say, "We're seeing significant increased uptake of our radioactive sugar in this lymph node." As a result, we might suspect cancer.
Carrying the example a little bit further, let's say the lung cancer patient presents with a tumor. The next question is should this patient have surgery or not? If the patient is going to have radiation therapy, to what region should he or she have radiation therapy? In about one out of three of these individuals, when a PET scan is added to the information, it changes whether the patient is a candidate for surgery, or what sort of field (tumor and surrounding tissues) should receive radiation.
Now, that is just looking at PET images alone-without what we call "fusion" imaging.
Q. What is fusion imaging?
Again, you have anatomical imaging and functional imaging. In the past, all this information was "eyeballed," with the radiologist looking at the anatomical images and the nuclear medicine specialist looking at the functional images and the clinician in between, and they were all trying to figure out what was going on. You might look at the anatomical imaging one week and your nuclear medicine functional imaging a week later – then everybody put their heads together to come up with an answer.
About five years ago, some of the first commercial equipment became available for "fusing" functional data with anatomical data. The advantage of examining anatomical data and functional data together is that it pinpoints the exact location of the tumor and enhances diagnostic certainty.
One of our strongest tools for cancer management is called PET/CT – a combination of a high-quality CT scanner and a PET scanner. If the patient has a CT scan on scanner A and a PET scan on camera B, you can often use computer software to fuse the two, but the combination PET/CT scanner reduces imaging time by about half. That makes it more comfortable for the patient. In addition, the PET/CT improves accuracy because everything is done on the same table. Finally, the fused images are available quickly for interpretation, instead of requiring an extra processing step with a computer.
Q. So what's the bottom line?
Interpretation of fused images is better than separate interpretation of either PET or CT images standing by themselves. It allows us to help physicians treat tumors earlier by providing a more specific, comprehensive picture of the disease.