Following 10,273 women who had the most common type of breast cancer, researchers examined outcomes for the 69 per cent of patients who scored between 11 - 25 on the test.
Judy Perkins, an engineer from Florida, was 49 when she was selected for the radical new therapy after several rounds of routine chemotherapy failed to stop a tumour in her right breast from growing and spreading to her liver and other areas.
The study, which was published this week in the New England Journal of Medicine, was funded by the National Cancer Institute, a number of grants and foundations, as well as proceeds from the the United States breast cancer stamp. Generally, after surgery, such patients receive endocrine therapy, such as tamoxifen, which is created to block the cancer-spurring effects of hormones.
"The study should have a huge impact on doctors and patients", Albain said.
Chemotherapy may be avoided in about 70% of early-stage breast cancer patients, thus limiting chemotherapy to the 30% for whom it can be predicted to be beneficial, a study released on Sunday (3 June) shows.
"It feels miraculous, and I am beyond amazed that I have now been free of cancer for two years", Perkins said.
This means in practice doctors can tell 70 percent of these patients they don't need to agonize over whether to get chemo, says study co-author Dr. Kathy Albain, a hematology and oncology professor at Loyola University Medical Center in IL.
Previously, the challenge doctors and patients faced was a mid-range score.
"If confirmed in a larger study, it promises to further extend the reach of this T-cell therapy to a broader spectrum of cancers", he said.
To find out if the cancer could grow or spread, a sample of the tumour is tested after surgery for 21 genetic markers.
Many doctors have been using that test or similar ones for years.
However, certain women 50 or younger will still benefit from having chemotherapy.
CAR-T only works in blood cancers, Rosenberg said.
In this latest study, researchers focused on the 69 percent of the clinical trial's participants who had mid-range test scores. Women were separated into 2 groups, one receiving only hormone therapy, the other receiving chemotherapy and hormone therapy.
Simon Vincent, director of research at Breast Cancer Now, added: "This is a remarkable and extremely promising result, but we need to see this effect repeated in other patients before giving hope of a new immunotherapy for incurable metastatic breast cancer". Erring on the side of caution, doctors usually give these women chemotherapy.
More than two years on, she is cancer-free and says she has "gone back to normal everyday life", even enjoying strenuous hiking and kayaking trips.
The findings only apply to women with specific early stage breast cancers. No less daunting than the diagnosis itself was the prospect of chemotherapy and its associated side-effects - hair loss, nausea, etc.
All the same, he says, the message of the new study isn't that chemotherapy is irrelevant. "But we need to be precise on when to use it and who to recommend it to". Genes are either off or on. They used gene testing of the tumor to determine risks and treatment.
The technology is a "living drug" made from a patient's own cells at one of the world's leading centres of cancer research.
Brawley noted that the reduction in chemotherapy might reduce revenues for some oncologists, but he doesn't think they'll care.