A trial of more than 10,000 women - including 690 Irish women - with the most common form of early breast cancer found the treatment was unnecessary for many after surgery.
More than 20,000 women in the United Kingdom are diagnosed with hormone-receptor positive, HER2-negative, node-negative breast cancer annually.
Dr Aranda said despite the positive results of the study, some women would still require chemotherapy to treat their cancer, particularly younger women.
Professor Keane said the test will identify which women will benefit from endocrine (hormone) therapy only.
As always, more research will clarify these findings, but for now, this new information will likely save thousands of women the pain and long-term health toll of chemo, while not at all affecting their chances of beating cancer.
And while celebrating this genuine advance, we should remember just how far we have to go in finding effective therapies for metastatic breast cancer, and other breast cancer subtypes for which treatment options are still limited.
The cancer had spread to various parts of her body, including the liver.
"You shouldn't do chemo unless you have to", added Dr. Jack Jacoub, medical oncologist and medical director at the MemorialCare Cancer Institute at the Orange Coast Medical Center in California.
This decline likely came about thanks to earlier research into gene testing, showing that chemo wasn't helpful for low-risk patients and of marginal benefits for others, says Baselga.
The genomic test for this type of breast cancer, called Oncotype DX, measures the expression of 21 genes in tumor tissue removed at the time of surgery and predicts risk of recurrence on a scale of 0 to 100. Some study leaders consult for breast cancer drugmakers or for the company that makes the gene test. Previous studies had made clear that women with low scores could skip chemotherapy and those with high scores should get it.
The new research now exempts women with scores between 11 to 25 from chemotherapy as well, unless they are premenopausal.
"They are attacking their own cancers", Rosenberg said in an interview.
The overall survival rate was similar: 93.9% for those who received hormone therapy alone and 93.8% for those who received both therapies.
After years of follow up, their results were almost identical with women who received hormone therapy yielding a 93.9 per cent survival rate and those with both therapies a 93.8 per cent survival rate.
Scientists found that giving hormone therapy alone was as good as the combination of chemotherapy and hormone therapy in women with the intermediate score.
'Every day, women with certain types of breast cancer face the awful dilemma of whether or not to have the treatment, without hard facts about the benefit for them'.
If doctors adopt the study's findings, a lot of women could be affected.
Breast cancer is the leading cause of cancer death in women worldwide, causing some 1.7 million new cases annually and over half a million deaths.
Carl June, an immunotherapy expert at the University of Pennsylvania, said the breast-cancer patient's response to TILs was "striking". The disease returned 10 years later, creating tumours throughout her liver and chest. A 2017 study showed that for breast cancer that hasn't spread to the lymph nodes, the rate of chemotherapy has dropped from around 27 percent in 2013 to 14 percent in 2015.
After growing billions of these immune cells in the lab, the researchers screened them to find which ones would most effectively find and destroy the woman's cancer cells by recognising their abnormal proteins. Her hair fell out, she developed an infection and was hospitalized for a low white blood count, "but it was over fairly quickly and I'm really glad I had it".