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New Tracer Fine-Tunes Prostate Cancer Screening

Radioactive carbon enhances PET scan

FRIDAY, Feb. 15, 2002 (HealthDayNews) -- Adding a tag of radioactive carbon to a crucial cell nutrient may help doctors find and treat prostate cancer more effectively.

In a new study, Japanese researchers say the marker, 11C-acetate, is more sensitive than conventional tracers used in prostate tumor imaging, turning up in 100 percent of patients' primary cancers and in all but one of their known metastases.

The marker helps doctors pinpoint tumors with positron emission tomography (PET), a computerized imaging method that detects abnormal cell activity. Since tumors grow faster than healthy tissues, cancers appear as hot spots on PET tests.

If CT and magnetic resonance imaging (MRIs) map organs, the PET scan is medicine's version of the wiretap. The technology has become increasingly useful as a way to both identify tumors and how they spread, and it helps doctors monitor the effectiveness of cancer therapy.

FDG, or fluorodeoxyglucose, is the standard marker for PET scanning. The substance mimics glucose, the power source that keeps tumors growing. However, while FDG is ideal at detecting rapidly dividing cells, it's not so good at finding more indolent cancers, such as tumors in the thyroid and prostate glands.

In the latest study, which appears in the February issue of the Journal of Nuclear Medicine, Dr. Nobuyuki Oyama and his colleagues compared 11C-acetate and FDG doped with 18fluorine in 18 patients with prostate cancer. Four others were screened with 11C-acetate alone.

Cells require acetate to construct their fatty outer walls, so like glucose it's a good gauge of their growth.

Prostate tumors took up more 11C-acetate than 18F-FDG, the researchers say, providing better images of the primary cancer sites. In addition, while FDG let the researchers see tumors in 15 of 18 patients, 11C-acetate appeared in all 22 men.

"When we look at the images, if the uptake of the tracer is higher, it's easier to detect the tumor," says Oyama, formerly of Fukui Medical University and now a radiology instructor at Washington University School of Medicine. "If the uptake is not high, sometimes it's difficult to differentiate cancer from normal tissue."

The new marker also hitched on to tumor cells that had spread from the prostate to lymph nodes and to bone, finding five of five and six of seven of those cancers, respectively. 18F-FDG, on the other hand, identified only two of the five cancers that had spread to lymph nodes, and only four of seven that had traveled to bone.

Dr. Dominique Delbeke, director of the PET center at Vanderbilt University Medical Center and associate editor of the nuclear medicine journal, calls the latest findings "exciting."

However, Delbeke adds, 11C-acetate does present some practical problems, foremost of which is its relatively short half-life, or the time it takes to decay by 50 percent. Whereas the half-life of 18F-FDG is about two hours, 11C-acetate's is only 20 minutes.

"You need to make it for each patient," Delbeke says, which leaves the PET machine idle and reduces the number of people who can be screened each day.

Dr. Abass Alavi, a nuclear medicine expert at the University of Pennsylvania, agrees the half-life problem is a significant one for scanning facilities trying to reduce the cost of PET per patient. It also forces hospitals to make the substance or find a nearby supplier, since losses during transport are so high.

If 18F-FDG can be tweaked to somehow locate aggressive tumors, it "may look better" to nuclear medicine centers, Alavi says.

Still, Alavi says PET "is going to be the way to go for many disorders of the body, including cancer." It boasts superior resolution to other scanning methods, and if the right marker can be developed, it will detect both fast and slow-growing tumors.

What To Do: To learn more about prostate cancer, try the University of Michigan or CAPCURE.

SOURCES: Interviews with Nobuyuki Oyama, M.D., Ph.D., radiology instructor, Washington University School of Medicine, St. Louis; Abass Alavi, M.D., chief, nuclear medicine, University of Pennsylvania Medical Center, Philadelphia; Dominique Delbeke, M.D., Ph.D., professor, director, nuclear medicine and PET, Vanderbilt University Medical Center, Nashville, Tenn.; February 2002 Journal of Nuclear Medicine
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