Expert clinicians select genomic assays with nuance. For a patient where a low-risk result is desired to avoid chemotherapy, Oncotype DX may be chosen as it tends to yield more low-risk scores. Conversely, MammaPrint may be used for a chemo-hesitant patient, as it is more likely to return a high-risk result.

Despite compelling data from trials like PATINA, some patients with ER+/HER2+ breast cancer refuse maintenance endocrine therapy due to side effects. This highlights a real-world gap between clinical trial evidence and patient adherence, forcing oncologists to navigate patient preferences against optimal treatment protocols.

Trials like TaylorX and MINDACT use genomic scores to identify patients with early-stage, HR+/HER2- breast cancer who won't benefit from adjuvant chemotherapy. This avoids significant toxicity for two-thirds to over 80% of patients who would have received it under older guidelines, without compromising their outcomes.

The SET index, which measures estrogen receptor activity, identified a subset of patients in the NSABP B42 trial who derived a massive 17% absolute improvement in 10-year breast cancer-free interval from extended letrozole. This contrasts sharply with the modest 3% benefit seen in the overall trial population.

For premenopausal patients with extensive nodal disease (e.g., N2), the clinical indication for chemotherapy is so strong that even a low-risk genomic score would not be enough to withhold treatment. This highlights the primacy of clinical staging over genomic data in certain high-risk scenarios.

Even within recent major clinical trials like HER2CLIMB-05, less than half of eligible hormone receptor-positive patients received endocrine therapy. This highlights a critical and widespread gap in clinical practice, as this treatment adds significant benefit.

The rapid evolution of clinical evidence is reflected in ASCO guidelines. In just one year (2022 to 2023), recommendations for ESR1 testing in HR+ metastatic breast cancer changed from having insufficient data to recommending routine testing upon progression, highlighting the pace of change in oncology.

With several new oral endocrine agents showing similar efficacy for ESR1-mutated breast cancer, clinicians' choices are driven by practical factors. Treatment selection hinges on tolerability (e.g., vepdegestrant's QT prolongation risk) and familiarity with an agent's real-world performance, rather than marginal differences in trial data.

A registry trial of 3,000 women showed that BCI results caused physicians to change their initial recommendations for or against extended endocrine therapy in 41% of cases. The genomic test also significantly increased patient confidence in their treatment decisions, demonstrating its real-world utility in shared decision-making.