Researchers say, the results allowed them to separate women who would benefit from chemotherapy due to a higher risk of recurrent breast cancer, and those who tested at a lower risk and do not need chemo for treatment.
Experts believe the case, discussed at the American Society of Clinical Oncology meeting in Chicago, marks the start of a breakthrough for thousands of women who now have no hope.
Dr. Jennifer Litton at MD Anderson Cancer Center in Houston, agreed, but said, "Risk to one person is not the same thing as risk to another".
"[The findings] are both important and significant, and also practice-changing", says, Dr. José Baselga, a medical oncologist and physician in chief at Memorial Sloan Kettering Cancer Center in NY, who was not involved with this research.
"It means we can better use the treatments... so that we can achieve our ultimate goal", Dr. David Cescon of Toronto's Princess Margaret Cancer Centre told CTV News.
Some of those women have already been skipping chemotherapy based on less rigorous research.
This means in practice doctors can tell 70 percent of these patients they don't need to agonize over whether to get chemo, says study co-author Dr. Kathy Albain, a hematology and oncology professor at Loyola University Medical Center in IL.
Thousands of women have been tested over the years using Oncotype DX to help determine the true effectiveness of chemo. "The trial was created to address this question, and provides a very definitive answer".
The study applies only to women with breast cancers that are "hormone receptor-positive" - meaning they are fuelled by estrogen and progesterone - and "HER2-negative", and that have not spread to lymph nodes under the armpits.
Oncotype DX is becoming more standard.
These were divided into groups that will receive chemotherapy followed by hormonal therapy or hormonal therapy alone. The results showed there might be a small benefit from chemotherapy.
Researchers studied their outcomes, including whether or not cancer recurred, and their overall survival. Women older than 50 with a midrange risk - defined in the study as a score of 11 to 25 on a tumour test - can skip chemo and just have endocrine therapy. It looks for a genetic "signature" in a sample of the tumour and gives a score between 0 and 100, which can help to direct treatment decisions.
You'd think if a patient diagnosed with breast cancer had the option of undergoing chemotherapy or safely going without it, the answer would be obvious.
"Being able to say to a patient that truly there's no benefit for chemotherapy in your case, based on a test of your cancer, that's a very powerful intervention", Burstein added in an interview.
Now, after an infusion of billions of cancer-killing immune cells, and having been given a life expectancy of just a few months, there is no sign of cancer in her body - a "complete, durable regression" in the words of the scientists that developed the therapy.
The study enrolled 10,273 women, of whom 9719 with follow-up data were included in the main analysis set; 6711 women (69%) had an intermediate recurrence score of 11-25, while 1619 (17%) had a low recurrence sore of 10 or less and 1389 (14%) had a recurrence score of 26 or higher. I also, then, lost my hair.
For the youngest participants, chemotherapy was only slightly more effective for women with scores between 16 and 25.
"We are thrilled by this early finding, but we must remember that this type of immunotherapy remains an experimental approach that has a long way to go before it might be routinely available to patients".