Genetic Test Could Let Two-Thirds of Breast Cancer Patients Safely Skip Chemotherapy

For decades, oncologists have faced a question without a clean answer: which breast cancer patients truly need chemotherapy, and which would do just as well without it? The drugs save lives, but they exact a price—months of hair loss, nausea, fatigue, infection risk, early menopause, possible infertility, and long-term damage to the heart and nervous system. A large clinical trial presented this spring at the American Society of Clinical Oncology's annual meeting in Chicago suggests that genetic testing of tumor tissue could help answer that question with real precision, potentially sparing two-thirds of certain breast cancer patients from chemotherapy without compromising their survival.

The test, called Prosigna, works by measuring the activity of cancer-related genes inside tumor cells. Rather than relying solely on what a pathologist sees under a microscope—the tumor's size, whether it has spread to lymph nodes—the test calculates a score that reflects how aggressive that particular cancer is and how likely it is to return after standard treatment. A low score suggests the cancer is less aggressive and that surgery, radiation, and hormone tablets alone are sufficient. A high score signals greater risk of recurrence and indicates that chemotherapy is likely to make a meaningful difference. In essence, the test separates patients into two groups: those for whom chemotherapy will genuinely help and those for whom it would cause serious harm without real benefit.

The evidence behind this shift comes from the Optima trial, a large, randomized study led by researchers at University College London and involving multiple hospitals and cancer centers. More than 4,400 patients aged 40 and older with early-stage, hormone-driven breast cancer were enrolled—making it one of the largest studies of its kind. Half were assigned to standard treatment decisions based on traditional clinical factors; the other half had their chemotherapy recommendations guided by the Prosigna test result. Researchers followed both groups for several years to track cancer recurrence and survival.

The results were striking. More than two-thirds of women who would traditionally have been considered candidates for chemotherapy based on tumor size or lymph node involvement had low Prosigna scores, meaning they could safely skip the drugs. The five-year survival rate for those who avoided chemotherapy was 93.7 percent, compared with 94.9 percent for those who received it—a difference of just over one percentage point. Researchers concluded this gap was small enough to be clinically insignificant, particularly given that patients who avoided chemotherapy were spared months of serious side effects for no meaningful loss of protection.

The Prosigna test itself is not new; it has already been approved and used in some centers. What has been missing until now is robust clinical trial evidence showing that using it to guide treatment decisions is as safe as current practice in a broad, real-world population. In the UK, the next steps involve detailed review by the National Institute for Health and Care Excellence, the body that assesses which treatments the NHS should fund. That process typically takes months, but the expectation is that it will accelerate and broaden access to Prosigna or similar tests for eligible patients, moving them from niche options to standard care.

This is not a story about eliminating chemotherapy. Roughly one-third of patients in the trial had high Prosigna scores, indicating their cancers were more likely to recur without chemotherapy. For them, the recommendation remains unchanged: chemotherapy plus hormone treatment, because the potential benefit in reducing recurrence is significantly greater. The test's real power lies in concentrating chemotherapy where it is most effective while protecting low-risk patients from being overtreated.

The trial's findings apply specifically to a common but defined group: people aged 40 and older with early-stage, hormone-driven breast cancer that does not overproduce a growth protein called HER2. Other breast cancer subtypes—triple-negative and HER2-positive cancers, for instance—behave very differently and are treated with different drug combinations. Younger patients under 40 present additional complexity; their tumor biology and risk patterns can differ, requiring more individualized treatment decisions. The test is not a one-size-fits-all solution, and it does not replace careful discussion with a specialist team about each person's overall risk and best treatment plan.

The shift toward genetic and molecular testing reflects a broader change in cancer care. Similar tests are already guiding treatment decisions in lung cancer, where specific DNA changes can be targeted with tablets instead of standard chemotherapy, and in bowel and prostate cancers, where biological tests help estimate spread risk and guide treatment intensity. As these technologies become cheaper and more studies support their use, similar approaches are likely to become common across many cancer types, helping to avoid both undertreatment and overtreatment. The principle is consistent: understand each tumor's biology in detail, then match the intensity and type of treatment to that biology rather than simply to the tumor's size or stage.

The caution that has long surrounded changes to standard cancer treatment—the fear of undertreating someone—has given way to a new confidence grounded in evidence. Overtreatment in cancer care has rarely stemmed from bad decisions but from uncertainty. Without reliable ways to identify who would truly benefit, doctors often defaulted to treating more people rather than risking harm through undertreatment. Many patients endured chemotherapy that made little or no difference to their outcome. The Optima trial provides the kind of large, randomized, real-world evidence that health systems need before making major changes to routine care. For the first time, patients with low-risk genetic scores can make informed decisions about whether to accept a tiny difference in survival odds in exchange for avoiding months of serious side effects.