Combined Imaging Sharpens View of Tumors
CancerWise - August 2007
By Eileen A. Ellig
Picture this: A day when physicians can determine if a person is predisposed to cancer by detecting the subtlest molecular changes within their DNA. It may be accomplished with imaging – something much less invasive than a tissue biopsy or blood test.
This ability may not be too far off. Already radiologists are seeing some intracellular changes that promote cancer by combining two imaging methods:
- Positron emission tomography (PET)
- Computed tomography (CT)
Used at many cancer centers nationwide, the combined method also has had an enormous effect on cancer treatment, says Donald Podoloff, M.D., head of the Division of Diagnostic Imaging and a professor in the Department of Diagnostic Radiology and the Department of Nuclear Medicine at M. D. Anderson.
“I haven’t seen anything in diagnostic imaging make this kind of an impact on the cancer patient since CT was introduced back in the 1970s,” Podoloff says.
Each method holds specific benefits:
CT – CT scans are X-rays that provide three-dimensional views of the body’s organs, bones and other tissues, as well as tumors. Compared to basic X-rays, CT scans can more precisely show the location of a tumor in relation to critical structures and determine the stage of disease.
PET, also known as FDG-PET – Fluorodeoxyglucose (FDG)-positron emission tomography (PET) involves computerized scans of the body conducted after fluorodeoxyglucose, a radioactive labeled form of glucose (sugar), is injected into a vein.
Tumors entrap and more readily absorb glucose than normal cells and show up as active bright spots on the PET scan, indicating their location and concentration throughout the affected organ, Podoloff says.
One drawback to using PET alone is it doesn’t provide anatomic landmarks like CT. But merge the two images and “you now have a very powerful tool that can complete the picture,” Podoloff says.
Partners in discovery
The benefit of merging PET and CT caught radiologists by surprise, says Reginald Munden, M.D., D.M.D., chair ad interim of M. D. Anderson’s Department of Diagnostic Radiology.
“I don’t think any of us were truly aware of the benefit until we started looking at the images and saw the added sensitivity and specificity they provided in detecting and localizing tumors,” Munden says.
This technology, he adds, is contributing greatly to “our ability to characterize tumors and image the very biomarkers our clinical colleagues are trying to target.”
Scientists trace cancer images
Much research is being conducted to develop molecular imaging agents that can trace whether a drug is actually reaching the tumor, being taken up into the cells and working.
“And we can do this not at a research bench with a microscope, as is the traditional approach, but non-invasively through repetitive imaging and close monitoring of therapies,” says Juri Gelovani, M.D., Ph.D., chair of the Department of Experimental Diagnostic Imaging.
One example involves the use of FDG to trace tumors during PET scans, to evaluate the early effects of Gleevec® in treating gastrointestinal stromal tumors (GIST).
Within 24 to 48 hours, “we can determine if there has been a response even though the mass is still there,” says Homer Macapinlac, M.D., chair of the Department of Nuclear Medicine and head of the PET imaging program at
M. D. Anderson. “If it’s no longer absorbing glucose, for instance, then we know the drug is working and that generally correlates with a good outcome. We hope to emulate this for other novel therapies in the early assessment of treatment response.”
A limiting factor is that while a good surrogate marker for active cancer, fluorodeoxyglucose doesn’t show proliferating (dividing) cells, which might be a better index of tumor activity than glucose. To date, this is the only federally approved radioactive tracer for use in PET imaging.
With Gelovani’s support, Macapinlac says, “we’ll soon have available to us tracers that are more specific in measuring these kinds of metabolic parameters.”
Teams on assignment
In fact, Gelovani’s team is developing tracers that can be used to predict or monitor the effectiveness of certain anti-cancer drugs.
For example, in collaboration with Steven Kornblau, M.D., associate professor in the Department of Stem Cell Transplantation and Cellular Therapy, Gelovani and his team have developed a tracer for monitoring transplantation of genetically modified T cells for treatment of leukemia.
While these T cells are thought to elicit a positive response, they also can later cause graft-versus-host disease, a serious immune reaction of the donor’s cells to the recipient’s.
The tracer can be injected multiple times to hopefully diagnose graft-versus-host disease before severe clinical symptoms develop. Armed with this early information, clinicians can administer drugs that can eliminate any diseased cells.
Better imaging means better outcome for patients
With a dozen or so tracers under development, Gelovani is optimistic that new imaging agents will revolutionize the diagnosis and treatment of the disease.
The development of these agents is imperative for not only individualizing therapies but also monitoring therapies.
“We have an opportunity to let patients know within days rather than weeks or months whether their treatment is working,” Macapinlac adds. “Having this information upfront provides patients with hope, the fervor to continue the treatment despite the side effects.”
- Division of Diagnostic Imaging (M. D. Anderson)
- Experimental Diagnostic Imaging (M. D. Anderson)
- Department of Stem Cell Transplantation and Cellular Therapy (M. D. Anderson)
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