Approximately 68 percent of women with clinically high-risk breast cancer who would receive chemotherapy under standard practice can safely avoid it without measurable impact on survival. The OPTIMA Phase 3 trial of 4,429 patients, presented on May 30, 2026 at the American Society of Clinical Oncology annual meeting in Chicago and published simultaneously in the Journal of Clinical Oncology, demonstrates this using a 50-gene test called Prosigna. For tens of thousands of patients annually worldwide, this means avoiding heart damage, nerve toxicity, and one of oncology's most invasive treatments.
Understanding high-risk breast cancer
Not all breast cancers behave the same. Approximately 75 percent of all breast cancer diagnoses are estrogen receptor-positive and HER2-negative, abbreviated ER+ HER2-. In these tumors, the hormone estrogen drives cancer cell growth, while the HER2 protein plays no role. Hormone therapies work well against such tumors. But for large primary tumors (30 millimeters or larger) or lymph node involvement, decades of practice dictated an additional chemotherapy course. Oncology classified this group as clinically high-risk.
Chemotherapy is toxic. Anthracyclines and taxanes, the chemotherapy drugs commonly used in breast cancer, damage heart tissue, cause polyneuropathies with lasting nerve pain and numbness in hands and feet in a substantial portion of patients, can impair fertility, and increase the long-term risk of secondary blood cancers. The fundamental question has always been: who truly needs this? Tumor size and lymph node involvement indicate anatomical spread but reveal little about a tumor's biological behavior.
What the Prosigna test measures
Prosigna is not a genetic test for inherited mutations like BRCA1 or BRCA2. It examines the activity of 50 genes in the tumor tissue itself, a measurement called gene expression. Two tumors that look identical under a microscope can behave biologically very differently: one grows aggressively and metastasizes early, the other remains indolent. Prosigna makes this difference measurable and condenses it into a numerical risk score, the ROR-Score (Risk of Recurrence). Patients with a score up to 60 are classified as genomically low-risk. This group was the focus of the OPTIMA trial.
What the OPTIMA trial shows
The OPTIMA Consortium, led by Professor Rob Stein at UCL Cancer Institute, recruited 4,429 patients between 2014 and 2023 from the United Kingdom, Norway, Sweden, Australia, New Zealand, and Thailand. All met high-risk criteria: either tumors 30 millimeters or larger or up to nine positive lymph nodes. Without the trial, they would have received chemotherapy as standard.
One arm received exactly that: chemotherapy plus subsequent hormone therapy. In the other arm, the Prosigna test determined treatment. Patients with an ROR-Score above 60 received chemotherapy. Those below received hormone therapy alone. This applied to 68 percent of patients in the test-directed group.
Five-year results: For women with low Prosigna scores, the disease-free five-year survival rate without chemotherapy was 93.6 percent, compared to 94.8 percent for those receiving chemotherapy. The difference of 1.2 percentage points remained within the pre-specified non-inferiority margin. Study authors state it plainly: chemotherapy prevents a maximum of two recurrences per 100 treated patients in this group, but adds toxicity to all 100. The European breast cancer patient network Europa Donna called the results an important step toward personalized therapy.
Comparison: How OPTIMA advances prior testing
Genomic tests for breast cancer have progressively changed clinical decision-making over the past 15 years. The TAILORx trial, published in 2018 in the New England Journal of Medicine with 10,273 women, showed that the Oncotype DX test allowed 70 percent of patients without lymph node involvement to safely avoid chemotherapy. This was a paradigm shift, but applied exclusively to women with node-negative tumors.
The RxPONDER trial (2021, also in the New England Journal of Medicine) expanded eligibility to women with one to three positive lymph nodes. Postmenopausal women in this group could skip chemotherapy. One major blind spot remained: women with four or more positive nodes or very large tumors were still routinely prescribed chemotherapy.
OPTIMA closes exactly this gap. The study was the first to include patients with up to nine positive lymph nodes and tumors over 30 millimeters, showing that 68 percent of women in this previously chemo-mandatory group achieve identical survival without the treatment. For a patient with four positive lymph nodes who would have received chemotherapy under standard practice, this represents a fundamentally different clinical recommendation.
When Prosigna becomes standard in clinical practice
The journey from trial data to routine clinical care is long. International oncology societies must evaluate such results through their guideline-setting processes. Updated Prosigna recommendations for the OPTIMA population are expected by fall 2026 at earliest from major consensus groups. The European Society for Medical Oncology has not yet integrated these findings into its guidelines. Some oncologists note that 1.2 percentage points difference in five-year survival may be clinically meaningful for a 45-year-old patient with decades of remaining life expectancy.
Reimbursement remains an open question. Prosigna costs between $2,500 and $4,000 per test depending on the laboratory. Coverage varies by healthcare system, and universal reimbursement for the OPTIMA population is not yet established. Globally, approximately 2.3 million women are diagnosed with breast cancer annually, with roughly 75 percent ER+ tumors. How quickly guideline bodies integrate OPTIMA data determines how many women will have the option to choose testing over standard chemotherapy rather than having the decision made for them.