Until now, women have faced a great deal of uncertainty about whether to add chemo to hormone therapy after a diagnosis with hormone-receptor positive, HER-2 negative breast cancer when found at an early stage before it has spread to the lymph nodes.
"Irish women contributed significantly to this trial and can be rightly proud of their input into improving care for future women who present with breast cancer", Prof Keane said.
The study was supported by the National Cancer Institute, the American College of Surgeons, Cancer and Leukemia Group B, NSABP Foundation, NCIC Clinical Trials Group, North Central Cancer Treatment Group, and Southwest Oncology Group.
Harold Burstein, a breast cancer specialist at Dana-Farber Cancer Institute, said that in some ways the debate over de-escalation misses a larger issue.
The study validates the gene test in the modern era and also clears up a "gray zone" - how to treat women with scores that were not clearly prognostic, Burstein said.
Very high scores are at increased risk of the cancer spreading, and those patients have benefited a lot by chemotherapy.
In doing so - they'll avoid the side-effects that come along with it.
"Our study shows that chemotherapy may be avoided in about 70 percent of these women when its use is guided by the test, thus limiting chemotherapy to the 30 percent of women we can predict will benefit from it", lead author Dr. Joseph Sparano said, via The Independent.
Patients under 50 with a score of up to 15 also do not need to undergo the treatment, the research concluded.
Those women should carefully discuss their options with their oncologist, said Brawley, because they would likely be candidates for the more aggressive, dual therapies.
The confusion comes from a genetic test called Oncotype DX that rates the likelihood of breast cancer recurrence on a 100-point scale.
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"It's going to save tens of thousands of people from having to take unnecessary chemotherapy", said Butler. Very low scores, up to 10, it's a very low chance that these patients get hormone therapy, and chemotherapy doesn't help them.
"Side effects, such as hair loss, severe pain and infertility, can be utterly devastating and linger long after they walk out the hospital doors".
Litton, the MD Anderson oncologist, said doctors need to consider each case on its own merits; she cautioned against ruling out chemo too quickly.
She had undergone a mastectomy in 2003 after the cancer was first diagnosed.
The current study involved more than 10,000 women and focused on those whose scores were in the middle range, from 11 to 25.
The overall survival rate was similar: 93.9% for those who received hormone therapy alone and 93.8% for those who received both therapies.
The most significant risk factors for breast cancer are gender (being a woman) and age (growing older).
"We'll give women in this group about six months of chemotherapy", Brawley said.
However, among younger women with scores 16 to 25, outcomes were slightly better in the chemotherapy group, so in those cases, doctors may urge patients to consider a chemo regimen.
The treatment involves a highly personalized approach and a patient's tumor is genetically analyzed to "identify the rare changes that might make the cancer visible to the immune system".