A study in Wednesday’s New England Journal of Medicine provides “a window into the future” of breast cancer therapy, experts say.
Researchers looked at a new way to track a cancer’s growth: measuring the amount of DNA that shakes loose from a cancer cell and floats freely in the blood, called circulating tumor DNA.
Doctors hope to use these measurements to assess how a patient’s tumor responds to treatment, according to the study, which included 30 women with advanced breast cancer and was led by the University of Cambridge in the United Kingdom.
While advanced breast cancer is not curable, it is treatable with drugs that can help some women live for years with their disease. When one drug stops working or proves too toxic, patients move on to another. Doctors assess how well these drugs are working by performing CT scans and other tests. But these scans, which measure the size or number of tumors, aren’t perfect. And they can take months to detect a problem, according to an accompanying editorial written by the University of Miami’s Marc Lippman and Baylor College of Medicine’s C. Kent Osbourne Circulating tumor DNA might give doctors and patients an earlier warning that a drug isn’t working, says Ben Park, associate professor of oncology at Johns Hopkins’ Sidney Kimmel Comprehensive Cancer Center in Baltimore. The tests might allow patients to stop taking a harsh drug that’s not working, sparing them months of side effects, Park says. Doctors could then prescribe an alternative drug, although there’s no evidence yet that switching drugs sooner would help people live longer. Because each breast tumor is unique — with a mixture of different genetic mutations — doctors would have to tailor each woman’s test based on the specific mutations in her cancer, Park says.Today, sequencing an individual cancer genome costs thousands of dollars. But experts expect that cost to fall sharply. Testing a patient’s circulating DNA “could easily turn out to be cheaper than some of the imaging tests we use now,” such as CT scans, says Eric Winer, director of the breast oncology center at Boston’s Dana-Farber Cancer Institute. Park hopes that doctors could eventually use circulating tumor DNA to help women with early-stage breast cancer as well, by giving them an early warning about a potential metastasis. The tests also could potentially speed up trials of new drugs, Park says.Park notes that a variety of tests based on circulating DNA are already on the market — not in cancer, but in obstetrics.These blood tests, based on “cell-free” fetal DNA, are offered to pregnant women as a non-invasive alternative to tests such as amniocentesis, which poses some risks to the fetus. The tests use fetal DNA circulating in the mother’s blood and can tell women early in pregnancy if they are carrying a child with a genetic condition, such as Down syndrome. Yet cancer patients shouldn’t expect their doctors to test their tumor’s circulating DNA anytime soon, Winer says. The Cambridge study was small and preliminary, and it’s too early to know if these tests will work as intended, he says. And while the tests may not be ready for prime time, doctors say the technique had great potential.To Wendy Woodward, associate professor of radiation oncology at Houston’s M.D. Anderson Cancer Center, “it’s a very interesting snapshot of what personalized medicine might look like in the future.”