New Test Refines Breast Cancer Treatment, Sparing Patients Extra Therapy

HOUSTON, TX – February 16, 2026 – For hundreds of thousands of breast cancer survivors, a difficult question looms after completing their initial five years of treatment: should they continue taking hormone-blocking drugs? New data suggests a powerful genomic test may finally provide a clear, personalized answer, identifying which patients are most likely to benefit from extended therapy while sparing others from years of unnecessary side effects.

At the 2025 San Antonio Breast Cancer Symposium, researchers presented a compelling new analysis of a landmark clinical trial. The findings demonstrate that the Endocrine Activity Index (EAI), a diagnostic tool from Houston-based Delphi Diagnostics, can pinpoint postmenopausal women with the most common form of breast cancer—hormone receptor-positive (HR+), HER2-negative—who derive substantial, life-altering benefits from continuing treatment beyond the standard five years.

The results address one of the most pressing dilemmas in modern oncology, offering a potential path to move away from a one-size-fits-all approach and toward a more precise, patient-centric standard of care.

The Challenge of Extended Endocrine Therapy

Endocrine therapy, which includes drugs like tamoxifen and aromatase inhibitors such as letrozole, has been a cornerstone of breast cancer treatment for decades. By blocking or lowering estrogen, these drugs effectively starve HR+ cancer cells of the fuel they need to grow. A standard five-year course of this therapy has been proven to dramatically reduce the risk of cancer returning.

However, the threat of late recurrence—cancer reappearing five, ten, or even fifteen years after diagnosis—remains a persistent fear for patients and a challenge for clinicians. To combat this, oncologists often consider “extended” endocrine therapy, continuing treatment for an additional five years.

The pivotal NRG/NSABP B-42 trial was designed to resolve this very issue. The study enrolled nearly 4,000 postmenopausal women who had completed five years of endocrine therapy and were disease-free. They were randomized to receive either an additional five years of the aromatase inhibitor letrozole or a placebo. The long-term results, updated after a decade of follow-up, showed a small but statistically significant benefit: extended letrozole therapy reduced the risk of breast cancer recurrence by about 3%.

While significant, this modest overall benefit presented a clinical quandary. The benefits must be weighed against the cumulative side effects of prolonged therapy, which can include bone loss, joint pain, and an increased risk of cardiovascular events. For many patients, the small chance of benefit may not outweigh the guaranteed impact on their quality of life. Until now, doctors have had few reliable tools to determine which of their patients belonged to the small group driving that overall benefit.

A New Tool for a Difficult Decision

The new analysis of the B-42 trial, presented by Dr. Eleftherios P. Mamounas on behalf of NRG Oncology, sought to apply a new lens to the trial's data using the Endocrine Activity Index (EAI) test. The EAI test, developed based on technology licensed from The University of Texas MD Anderson Cancer Center, analyzes a tumor's genetic signature to measure how active its estrogen and progesterone pathways are—in essence, how sensitive it is to hormonal signaling.

The results were striking. The test effectively stratified patients into two distinct groups.

Patients whose tumors had a high endocrine activity score (EAI ≥ 1.50) saw a significant and clinically meaningful benefit from taking letrozole for ten years instead of five. For this group, extended therapy resulted in a 7.1% absolute reduction in the chance of a breast cancer-free interval (BCFI) event over 10 years. In contrast, patients with a lower EAI score (<1.50) did not experience a statistically significant benefit from continuing treatment.

The effect was even more pronounced in a high-risk subgroup. For patients who had cancer in their lymph nodes at diagnosis and a high EAI score, extended therapy led to a remarkable 10.5% absolute reduction in recurrence events compared to placebo.

“Data from the B-42 study supports the hypothesis that longer durations of endocrine therapy are most effective for patients whose cancers are highly endocrine-sensitive,” said Delphi Diagnostics Chief Medical Officer, Federico A. Monzon. “This data continues to establish EAI as an important signature that can provide patient-specific insights to support treatment decisions.”

Navigating a Competitive Diagnostic Landscape

Delphi Diagnostics' EAI test enters a competitive and rapidly evolving field of genomic diagnostics aimed at personalizing cancer care. It is not the first tool to be applied to the question of extended endocrine therapy. Other established genomic assays, such as the Breast Cancer Index (BCI) and Agendia's MammaPrint, have also been used in re-analyses of the B-42 trial data to identify subgroups who benefit from longer treatment.

While these tests offer valuable prognostic information, Delphi Diagnostics argues its test is unique. “EAI is the only test that can provide insight into how active the estrogen/progesterone pathway of a breast cancer tumor is,” Dr. Monzon stated in the company’s press release. This focus on the underlying biological mechanism of endocrine sensitivity, rather than just general recurrence risk, could be a key differentiator.

The emergence of multiple predictive tools highlights a major shift in oncology. The question is no longer if genomic profiling should be used, but which test provides the most actionable information for a specific clinical decision. The ultimate adoption of the EAI test will depend on its performance against these competitors, its integration into clinical workflows, and its inclusion in influential treatment guidelines from organizations like the National Comprehensive Cancer Network (NCCN) and the American Society of Clinical Oncology (ASCO).

From Lab to Clinic: The Path Forward

The promising data presented at SABCS is a crucial step for Delphi Diagnostics, but it is just one milestone on the long road from laboratory innovation to widespread clinical use. The company, which holds an exclusive license for the technology from MD Anderson, now faces the challenge of commercialization.

This involves not only marketing to oncologists but also navigating the complex processes of regulatory approval and securing reimbursement from insurance payers. Payers will require robust evidence of clinical utility and cost-effectiveness before agreeing to cover the test.

The development of the EAI test also highlights the increasingly common and complex relationship between academic research institutions and private industry. The technology was developed in the laboratory of Dr. W. Fraser Symmans at MD Anderson. According to a disclosure, Dr. Symmans has a personal financial relationship with Delphi, which is managed by MD Anderson's Conflict of Interest Committee. Such relationships are standard in the translation of academic discoveries into commercial products and are managed through institutional policies to ensure research integrity.

For patients and their doctors, the arrival of tools like the Endocrine Activity Index signals a hopeful future. The ability to tailor the duration and intensity of therapy based on a tumor's specific biology promises not only to improve outcomes for those at highest risk but also to relieve thousands of other survivors from the burden of overtreatment, enhancing their long-term quality of life.