Penn Researchers Find That PET Imaging Sheds More Light On Complex, Microscopic Cancers

(Philadelphia, PA) -- Managing cancer is challenging for physicians in and of itself, but with certain types of cancer, diagnosis is difficult as well. In cancers based in unusual locations, such as in the lining of the lung (mesothelioma) or in the lymph nodes, conventional imaging technology cannot detect malignant cells or differentiate them from benign, or non-cancerous, ones. In a multi- disciplinary study evaluating malignant mesothelioma at the University of Pennsylvania Medical Center, researchers have shown the potential advantages of using positron emission tomography (PET) over structural imaging, including conventional X-Ray, magnetic resonance imaging (MRI), and computerized tomography (CT). These findings appear in the September issue of Chest, the official journal of the American College of Chest Physicians.

In contrast to structural radiologic techniques, PET technology is based on cellular metabolism -- the chemical reaction that drives the human body. PET imaging is non-invasive and is obtained as patients lie prone in a tube-like structure while detectors take readings of the body. A tracer agent is injected prior to the scan and binds to the organ or diseased sites. As a result, the tumorous area is highlighted with bright points of light on a PET computer screen, depicting clusters of malignant cells. PET imaging is sensitive enough to detect cancer cells in lymph nodes smaller than one centimeter. Other imaging technology may not recognize these microscopic tumor cells until they were larger -- perhaps two or more months later. Moreover, PET can provide images soon after treatment to show physicians whether the therapy is effective or not.

"Consider the benefits of using PET imaging with life-threatening, microscopic cancers," says Abass Alavi, MD, co-investigator of the study and Chief of the Nuclear Medicine division at Penn. "This technique can detect tumors well before structural scans are able to identify them, can accurately determine the extent and exact area of the body where tumorous cells are active, and can show treatment response soon after it is administered."

In this study, a minimally-radioactive tracer agent called 2-Deoxy- 2(18F)fluoro-D-glucose, or FDG, was administered in 28 patients before PET scanning. FDG is sensitive to the increased glucose metabolism observed in malignant tumor cells, generating points of white light on PET scans to represent areas of active tumor. The results of PET imaging were compared to those of CT scans, video-assisted thoracoscopy (examination of the chest cavity with an endoscope), and/or surgical biopsies. The FDG-PET images differentiated malignant and benign disease, achieving ninety-one percent (91%) sensitivity in detecting the tumors and one-hundred percent (100%) specificity between cancerous and non-cancerous cells in these patients. FDG-PET images provided excellent delineation of the active tumor sites. Metabolic lymph node activity was found in the FDG-PET images of 12 patients, nine of which appeared negative on CT scans.

"PET scans recognize malignant tumors that may go unnoticed by X-Ray, MRI, or CT scans," explains Daniel Sterman, MD, co-investigator and director of Penn's Interventional Pulmonary Program. "This technology allows us to make more precise assessments without the use of contrast dyes or invasive instruments."

Mesothelioma is primarily associated with exposure to asbestos and is characterized by the appearance of tumorous cells in the lining of the lung and chest cavity. The incidence of malignant mesothelioma is expected to rise until the year 2000, and will remain substantial for the next three decades. Mesothelioma cases are on the rise in women and those with no asbestos exposure as well. In difficult cases of microscopic cancers such as those in the lining of the lung or in the lymph nodes, conventional imaging cannot clearly establish malignancy. PET imaging can pinpoint the areas of disease activity, as well as evaluate response to treatment, to provide crucial information for disease management.