Background: Expression-based molecular subtypes of breast cancer (BC) predict tumor behavior and therapeutic response. Subtype distributions by age and sociodemographics can inform strategies for BC screening, treatment, and prognosis. The conventional approach, adopted by NCI’s Surveillance, Epidemiology, and End Results (SEER) Program, uses HR and HER2 to label: “triple negative” (HR-HER2-), “HER2-enriched” (HR-HER2+), “luminal A” (HR+HER2-), and “luminal B” (HR+HER2+). However, immunohistochemical (IHC)-based receptor labels may not reflect clinically and epidemiologically relevant molecular subtypes that share the same nomenclature, e.g., luminal B.

‍Methods: We compared IHC labels by HR/HER2 to molecular subtypes by MammaPrint (MP) and BluePrint (BP) for patients in the phase II neoadjuvant I-SPY2 TRIAL for high-risk, stage II-III BC (NCT01042379, n = 981) and in the multicenter, prospective FLEX Registry for stage I-III BC (NCT03053193, n = 5,679).

‍Results: IHC labels were discordant with MP/BP in 52% of I-SPY2 and 43% of FLEX cases (Table 1). HR-HER2- had the highest concordance with basal-type (99% in I-SPY2, 88% in FLEX). HR+ labels had the least agreement with MP/BP: HR+HER2- tumors were molecularly luminal B and basal in 71% and 29% of I-SPY2 and 40% and 4% of FLEX cases, respectively. HR+HER2+ tumors were molecularly luminal A and HER2-type in 10% and 60% of I-SPY2 and 15% and 36% of FLEX cases, respectively. Of molecularly luminal B cases, only 14% in I-SPY2 and 7% in FLEX were HR+HER2+.

‍Conclusions: IHC markers collected by population-based registries (SEER) enable BC surveillance. However, IHC labels cannot be used as surrogates for molecular subtypes by MP/BP, especially for luminal B tumors. Given the unmet need to improve management of luminal B BC, we anticipate the growing importance of molecular subtyping to inform treatment and epidemiological research. We propose that the BC research community work with SEER to update its IHC labels to avoid overlap with molecular subtype nomenclature and incorporate such modern classifications when available.