“The study should have a huge impact on doctors and patients”, said study co-author Loyola Medicine oncologist Dr Kathy Albain.

"We knew we were overtreating a lot of women with chemotherapy, in our gut. We can de-escalate toxic treatments and do that with certainty."

The study was praised by cancer research advocates.

"I don't get optimistic about a lot of things, but I've been very optimistic about this," said Otis Brawley, chief medical officer of the American Cancer Society.

The findings offered patients and doctors peace of mind about forgoing chemotherapy and would prevent tens of thousands of breast cancer patients each year from the harmful side effects including hair loss, nausea, vomiting as well as an increased risk of leukaemia and congestive heart failure, Brawley said.

The study was centred on a 21-gene test performed on tumours that examines genes from a patient's breast cancer biopsy sample and allows doctors to assign a patient a "recurrence score".

Previous studies suggested that patients with low scores (10 or lower) did not need chemotherapy, while women with high scores (above 25) did need and benefit from chemotherapy. But there was no conclusive research determining whether or not a patient with a mid-range score should undergo chemotherapy.

"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 researchers found that for participants with gene test scores between 11 and 25 (particularly 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 and for younger women with scores 16 to 25, outcomes were slightly better in the chemotherapy group, the study reported.

Albain said researchers were now invesitgating what types of chemotherapy might be effective on various groups based on their genomics.

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.

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?' "

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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.

Director of the Comprehensive Cancer Risk and Prevention Clinic at University of Chicago Olufunmilayo Olopade said the study helped move forward the push for precision medicine.

The study also had 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.

"Now we're actually peeling back and not treating everyone the same way."

Chicago Tribune, AP