PET Scan Personalizes Cancer Treatment

DALLAS--Radioactive tracers can reveal whether a breast tumor is shrinking in response to the drug tamoxifen, according to results from a pilot trial announced here yesterday at a national meeting of the American Chemical Society. The findings suggest that tracers coupled with positron emission tomography (PET) could refine cancer treatment by ensuring that patients receive the most effective drug for a tumor soon after a diagnosis.

Physicians often start cancer patients on a relatively mild drug, then switch to more aggressive treatments if a tumor resists. The first line of attack for breast cancer, for example, is usually tamoxifen. The drug will shrink tumors in about two-thirds of patients--those whose cancer cells are studded with receptors for the hormone estrogen. But oncologists must wait weeks to months before checking if a tumor has grown to establish that a particular patient is not responding to tamoxifen.

Hoping to develop a faster method for doctors to find out if a tumor is resisting tamoxifen, chemist Michael Welch and co-workers at Washington University School of Medicine in St. Louis sought a quick way to check whether a tumor has estrogen receptors. They tagged estradiol, the body's own form of estrogen, with a short-lived radioisotope that can be tracked in the body using a PET scan. As a test, they injected the radioactive estradiol into 23 breast cancer patients before treatment with tamoxifen. Only those patients whose tumors bound the estradiol responded to the antiestrogen drug, Welch reported. He also described preliminary evidence of how a second radiotracer can distinguish rapidly growing tumors--which resist radiation therapy--from more vulnerable slow-growing tumors in humans.

Because radiotracers reveal subtle chemical changes in cancer cells, these methods could let researchers quickly know whether a drug is working, says chemist Ken Krohn, a professor of radiology and radiation oncology at the University of Washington, Seattle. The PET imaging techniques could also be used to test experimental drugs, he says.