Breast cancer patients whose tumors have spread to other parts of their bodies live from scan to scan. Is their treatment working? Or will they learn their cancer is growing again?

But a new study sponsored by the drug company AstraZeneca showed that there is an alternative: Instead of waiting for a scan to show that a cancer is growing, it’s possible to find early signs that the cancer is resisting the drugs that were controlling it.

To do that, researchers used a blood test to find mutations in cancer cells that let the tumors defy standard treatments. Early detection allowed patients to be switched to a different drug that overcomes the mutated cancer. The result was to keep the cancers in check longer, and allow patients to have more than an extra year without deteriorating quality of life.

The study was reported Sunday at the annual meeting of the American Society of Clinical Oncology and published in The New England Journal of Medicine.

Breast cancer specialists who were not associated with the study applauded the results, saying blood tests could transform the way they monitor patients.

“This is a paradigm change,” said Dr. Mary Disis, professor of medicine and oncology at the University of Washington and the Fred Hutchinson Cancer Center.

Dr. Lisa Carey, deputy director of clinical sciences at the Lineberger Comprehensive Cancer Center at the University of North Carolina, said the results are “practice changing.”

But the list price of drugs that could be used to treat patients in these scenarios will be high — probably around $24,000 a month — until generic versions are manufactured.

The implications of the study go far beyond breast cancer, said Dr. Norman Sharpless, former director of the National Cancer Institute and now professor of cancer policy and innovation at the University of North Carolina School of Medicine.

The blood test, which detects minuscule amounts of DNA shed by cancer cells, can be used in other cancers, he said.

“Blood-based diagnosis is really going to transform therapy,” Sharpless said. “Remission is no longer going to be a passive process,” with anxious patients going in for scans every six months. Instead, he predicted, blood tests will allow doctors to squelch drug-resistant cancers before the tumors have a chance to grow.

The new study involved 315 patients with advanced breast cancer, meaning it had spread beyond their breasts. In that situation, the cancer cannot be cured. The goal is to contain it and to allow patients to live longer without debilitating symptoms.

All had the most common type of breast cancer, which is fueled by receptors that latch onto estrogen to spur its growth.

All were taking the standard treatment. One drug, an aromatase inhibitor, reduces the amount of estrogen in the body. A second drug disrupts cell division. Together, Disis said, the drugs provide “a double whammy that can really control and suppress metastatic breast cancer.”

None of the patients had any evidence on scans that their cancers were progressing. But eventually, the cancers were expected to mutate and defy the standard drugs, often by altering their estrogen receptor so it signals the cancer cells to grow without ever binding to estrogen.

To find those mutations early, the patients had blood tests every two to three months to look for minute amounts of mutated DNA from cancer cells.

When those mutations were detected, half of the patients were randomly assigned to continue with their standard treatment and half were switched to an experimental drug, camizestrant.

The drug, taken as a pill, is an estrogen receptor degrader: It destroys the receptor on cells so it cannot spur growth.