Real-world data demonstrates that a subset of node-negative (N0) breast cancer patients with high-risk features has a recurrence and mortality rate nearly identical to that of node-positive (N1) patients. This finding justifies intensifying adjuvant therapy with agents like CDK4/6 inhibitors for this seemingly lower-risk group, as was done in the NATALEE trial.

For high-grade, PR-negative ER+ breast cancers with very high Ki-67, oncologists may use intrinsic subtyping assays. If a 'basal-like' subtype is identified, they consider treating the patient with a triple-negative breast cancer regimen (like KEYNOTE-522) instead of standard ER+ therapy, a non-standard but biology-driven approach.

Cancers with estrogen receptor (ER) expression of 50% or less, while technically HR+, often behave biologically like basal or triple-negative tumors. These cancers are not primarily endocrine-driven and show a significant benefit from the addition of immune checkpoint inhibitors, challenging traditional subtype classifications.

Clinicians are moving beyond strict immunohistochemistry cutoffs for treatment decisions. Tumors with low estrogen receptor expression (ER-low, <10%) are often considered not to be primarily estrogen-driven and are treated with immunotherapy-based regimens standard for triple-negative disease, reflecting a shift toward biologically-informed therapy.

Beyond being a simple biomarker for oral SERD eligibility, clinicians are recognizing that the ESR1 mutation is a 'bad actor' associated with more aggressive disease. It increases cancer cell adhesion, makes circulating tumor cell clumps larger, and promotes liver-tropism, underscoring the need for early and effective intervention.

The KEYNOTE-756 and Checkmate 7FL trials found high pathological complete response (pCR) rates with neoadjuvant immunotherapy in ER-low (1-10%) breast cancers. This suggests this unique subgroup, often excluded from triple-negative trials but behaving similarly, may benefit significantly from immunotherapy, though it is not yet standard of care.

The degree of hormone receptor positivity can guide maintenance strategy. In patients with strongly ER/PR-positive tumors, an endocrine-based approach like the PATINA regimen may be favored. Conversely, in low ER/PR expressing tumors, a more HER2-focused regimen could be more effective.

It is crucial to re-biopsy metastatic breast cancer upon progression because the tumor's fundamental biology can change. A patient initially diagnosed with ER-positive disease can develop triple-negative, PD-L1 positive disease, which opens up entirely new treatment options like immunotherapy and different ADCs.

Experts are more cautious about sequencing ADCs back-to-back in triple-negative breast cancer (TNBC) compared to less aggressive subtypes. A sobering 50% of TNBC patients do not receive treatment beyond the first line, making it critical to use the most effective therapy upfront rather than saving options for later.