Chemotherapy, though it’s capable of slowing down cancer, isn’t pretty. It may be the difference between recovering or not for some cancer patients, but others do not reap any benefits from the aggressive radiation.

A new study published in the New England Journal of Medicine suggests that a 21-gene test done on a breast cancer tumor can identify which patients qualify to safely avoid chemotherapy. The study provides the best evidence yet that a multigene test can spare patients from undergoing chemotherapy when it wouldn’t actually be beneficial.

The trial involved 10,253 women with breast cancer that hadn’t yet spread to lymph nodes, but still contained features indicating they should probably undergo chemotherapy. The women were given tests that examined 21 different genes from these tumor samples, and then assigned scores between 0 and 100. The lower the score, the less likely the cancer would recur in other organs if only treated with a pill, such as tamoxifen, rather than chemotherapy. Previous studies had found that a low score suggested chemotherapy did not add any survival benefit to tamoxifen treatment.

For the trial, women whose tumors scored 10 or lower only received hormone therapy rather than chemotherapy. They were followed closely for the next five years, and researchers found that there was less than a 2 percent risk of their cancer spreading to other organs. The overall survival rate was 98 percent.

“We knew these patients were going to do well, but we didn’t dream they would do this well,” study lead author Joseph Sparano told The Wall Street Journal.

The trial is an important step toward avoiding a treatment with harmful side effects. “This should provide a lot of reassurance to women and their physicians,” said co-author Dr. Kathy Albain, an oncologist at Loyola University Medical Center and Loyola University Chicago Stritch School of Medicine, in a press release. “In women whose breast cancer scored low on the multigene test, there was outstanding survival with endocrine therapy alone. The test provides us with greater certainty about who can safely avoid chemotherapy.”

More follow-up research will be needed, the researchers said, in order to determine whether women with scores in the intermediate range can safely avoid chemotherapy in favor of hormone treatment.

Source: Sparano J, Albain K, et al. Prospective Validation of a 21-Gene Expression Assay in Breast Cancer. New England Journal of Medicine. 2015.