Karen Bonham joined the OPTIMA trial knowing her breast cancer diagnosis meant potential months of chemotherapy—but the gene test changed everything. The Prosigna test, which measures activity of genes involved in breast cancer growth, revealed her tumor carried a low risk score. That meant she could skip chemotherapy entirely and treat her cancer with hormone therapy alone, avoiding the physical toll and long-term side effects that often accompany the drugs. Her experience reflects a sea change in breast cancer care: a landmark international trial has found that more than two-thirds of patients aged 40 and over can safely forgo chemotherapy without increasing their risk of recurrence.
The discovery matters because chemotherapy, while effective overall at lowering recurrence risk, carries real costs. Clinicians have long suspected that many patients with hormone-sensitive breast cancer—the most common type—receive little or no benefit from the drugs but endure their significant and sometimes dangerous side effects anyway. The OPTIMA trial, led by Professor Rob Stein at the UCL Cancer Institute, was designed to solve this puzzle by using tumor biology rather than traditional clinical features to guide treatment decisions.
The trial followed 4,429 people across the UK, Norway, Sweden, Australia, New Zealand, and Thailand—all with early-stage, hormone-sensitive breast cancer that had spread to nearby lymph nodes. Participants were randomly assigned to either standard chemotherapy followed by hormone therapy, or to a test-directed group where the Prosigna test determined their path. Those with a low score (60 or below) received hormone therapy alone; those with a high score received both chemotherapy and hormone therapy.
The results vindicate the approach. Among the 68 percent of participants with a low Prosigna score, outcomes were strikingly similar five years later: 94.8 percent of those who received chemotherapy alongside hormone therapy were alive and free from cancer recurrence, compared to 93.6 percent of those treated with hormone therapy alone. That 1.2 percentage point difference sits well within the 3 percent threshold that patients and clinicians had agreed was acceptable. The trial was designed to test whether avoiding chemotherapy would lead to meaningful harm—and it did not.
What makes this breakthrough particularly significant is its accessibility. Unlike some genomic tests, Prosigna can be run by NHS laboratories with the appropriate equipment, meaning the findings have immediate practical application across Britain's health system. The test itself requires only a tissue sample from surgery or even a diagnostic needle biopsy, making it straightforward to implement.
"OPTIMA addresses a long-standing challenge in breast cancer care: identifying who truly benefits from chemotherapy and who does not," Professor Stein said. The implications ripple outward in both directions. For patients, especially those aged 40 and over, this means many can now avoid months of treatment without compromising survival. For health systems stretched thin, it represents more efficient, evidence-based use of resources—and a reminder that sometimes the most powerful medicine is knowing when not to use it.
The findings, presented at the 2026 American Society of Clinical Oncology meeting, promise to transform care for thousands of patients each year.