For women 50 or younger, chemotherapy is unwarranted for those with an Oncotype score under 16 - about 40 percent of breast cancers in this age group, the researchers said. Of those women, 6,711 had test scores between 11 and 25, which placed them in the intermediate risk group.
The hormone-blocking drug tamoxifen and related medicines, called endocrine therapy, have become an essential part of treatment for most women because they lower the risks of recurrence, new breast tumors and death from the disease.
She had undergone a mastectomy in 2003 after the cancer was first diagnosed. The results suggest that nearly 70 per cent breast cancer patients can avoid chemotherapy.
The new treatment finds T-cells that are successfully killing the patient's tumours and manufactures billions of clones in the laboratory.
"We can spare thousands and thousands of women from getting toxic treatment that really wouldn't benefit them", said Dr Ingrid A Mayer, from Vanderbilt University Medical Center, an author of the study. "Not only did these trials provide great relief for women in Ireland who participated and avoided three months of chemotherapy, it delivered considerable cost savings to the HSE", Ms Mulroe noted.
According to the Centers for Disease Control and Prevention (CDC), about 236,000 women and 2,100 men were diagnosed with breast cancer in 2014, the most recent year for which figures are available.
Albain said research is ongoing, as scientists are now exploring questions about what types of chemotherapy might be effective on various groups based on their genomics, and what will happen to the participants of the clinical trial as even more time passes.
The Oncotype DX genetic test has been available on the NHS since 2013, but the National Institute for Health and Care Excellence (Nice) is now updating its guidance on whether it should be recommended for use.
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"That's obviously huge for the individual patient".
The study, which has been closely followed by many oncologists worldwide who have been using earlier findings to guide their treatment of patients since the study began in 2006, was praised by cancer research advocates as progress made possible by cutting-edge advances in genomics.
In some cases, the results will go on to change how patients are treated. Doctors know that most don't need it, but evidence is thin on who can forgo it.
About 85% of breast cancers occur in women who have no family history of breast cancer.
Although this work does indicate that thousands of women can be spared this hard treatment, it doesn't mean skipping chemo is the right choice for all early-stage breast cancer patients.
FILE - In this January 28, 2003, file photo, Rep. Stephanie Stuckey Benfield, D-Decatur, promotes the breast cancer prevention stamp from the well on the floor of the House in Atlanta.
Well, we do this test of 21 genes.
Adine Usher, 78, who lives in Hartsdale, New York, joined the study 10 years ago at Montefiore and was randomly assigned to the group given chemo.
Knowing a patient's recurrence risk can spare them from enduring chemotherapy, but also can direct them towards it if they discover that they are at a higher risk, Sparano says.