Brawley said that in addition to offering patients and doctors peace of mind about forgoing chemotherapy, the findings would prevent tens of thousands of breast cancer patients each year from the debilitating and harmful risks associated with the treatment, including hair loss, nausea, vomiting and being at risk for leukemia and congestive heart failure.

"Chemotherapy is no Shangri-La," Brawley said. "We're saving people these side effects."

The study was centred on a 21-gene test performed on tumours that has been available for breast cancer patients since the early 2000s. The test examines genes from a patient's breast cancer biopsy sample and allows doctors to assign a patient a "recurrence score" from 0 to 100, according to a news release from Loyola Medicine announcing the findings.

While previous studies demonstrated that patients with low scores (10 or lower) did not need chemotherapy, and women with high scores (above 25) did require and benefit from chemotherapy, there was no conclusive research guiding patients and doctors on whether or not to undergo chemotherapy in cases with a tumour with a mid-range score, according to the study.

"It's a hard enough time for a woman, and they look at you and say, 'I want to do what you think is best,' and you have to say, 'unfortunately, you're in a group where there's uncertainty,' " Albain said. "Now I'm going to say, 'Hey, you don't need it. Look at these numbers; you're going to be fine.' "

The study enrolled 10,273 women who had the most common type of breast cancer (hormone-receptor positive, HER2 negative) that had not spread to lymph nodes.

Patients were randomly assigned to receive hormone therapy or chemotherapy, followed by hormone therapy.

The study found that for participants with gene test scores between 11 and 25 - especially among women ages 50 to 75 - there was no significant difference between the chemotherapy and no chemotherapy groups. Among women younger than 50, outcomes were similar when gene test scores were 15 or lower. Among younger women with scores 16 to 25, outcomes were slightly better in the chemotherapy group, the study reported.

Jennifer Mall, 47, was a patient of Albain's who agreed to participate in the study. She was shocked in 2010 when doctors found two tumours in one of her breasts during a mammogram. She hadn't felt any lumps or symptoms and had no family history of breast cancer before the diagnosis, she said.

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Mall underwent a double mastectomy and received hormone treatments, but decided not to go through chemotherapy after discussing the uncertainty of its effectiveness with Albain. But although Mall returned to health soon after her surgery and remains cancer-free today, she said she was haunted by the uncertainty of her decision.

"I worried that there were cancer cells left over in my body that were spreading everywhere because I didn't have the systemic treatment," Mall recalled. "Every time I had a pain or a cough in the winter and it just didn't go away, my mind would take me to: 'Maybe the cancer is in my lungs now?' "

Mall said while she, like many cancer survivors, will never completely be able to erase fear of recurrence, she is thrilled that thousands of women in her situation will be spared the anguish she experienced.

"I don't ever think I'll ever have the feeling that I'm completely out of the woods, but this definitely is exciting and makes me feel good," Mall said.

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.

Olufunmilayo Olopade, director of the Comprehensive Cancer Risk and Prevention Clinic at University of Chicago, said the study helps move forward the push for precision medicine - a philosophy of treating patients with the right medicine for their exact problems at a defined time. The study also has the potential to improve access to health care by keeping costs down for women who do not need chemotherapy, Olopade said.

"One of the challenges that we've had in breast cancer is we thought once size fit all, and everyone was getting too much treatment," Olopade said. "Breast cancer used to be that deadly disease that women were afraid of and they wanted everything done so they can actually survive. Now we're actually peeling back and not treating everyone the same way."

Cancer care has been evolving away from chemotherapy - older drugs with harsh side effects - in favour of gene-targeting therapies, hormone blockers and immune system treatments. When chemo is used now, it's sometimes for shorter periods or lower doses than it once was.

For example, another study at the conference found that Merck's immunotherapy drug Keytruda worked better than chemo as initial treatment for most people with the most common type of lung cancer, and with far fewer side effects.

Chicago Tribune, AP