The study, which analysed over 10,000 women, finally gave doctors an answer as to what to do when patients score an intermediate level of a genetic test.
While previous studies demonstrated that patients with low scores did not need chemotherapy, and women with high scores did need chemotherapy, there was no conclusive research on whether patients with a mid-range score should undergo the treatment.
Now it's been proven that patients in the mid-tier can be treated safely with just surgery and hormone therapy, BBC reports.
"Oncologists have been waiting for these results, it will affect practice on Monday morning", Dr Alistair Ring told the BBC.
"It's a fundamental change in the way we look after women with early breast cancer."
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,' " Loyola Medicine oncologist Dr Kathy Albain Albain said, reports SMH.
"Now I'm going to say, 'Hey, you don't need it. Look at these numbers; you're going to be fine.' "
Chemotherapy is often used after surgery to reduce the chance of breast cancer spreading or coming back, but side-effects range from vomiting, fatigue and infertility to permanent nerve pain. In rare cases, it can even lead to heart failure and leukaemia.
The results of the study will be implemented immediately.