Quantitative hormone receptors, triple-negative breast cancer (TNBC), and molecular subtypes: A collaborative effort of the BIG-NCI NABCG.

1008 Background: Most TNBC trials focusing on biology of the basal-like subtype (BLBC) allow borderline (1-10% staining) estrogen receptor (ER) and progesterone receptor (PgR) expression by immunohistochemistry (IHC); however the optimal ER and PgR cut points to enrich for non-luminal subtypes has not been studied. In this study,we compared quantitative ER/PgR status with gene expression-based intrinsic subtype in order to determine if borderline cases should be included in TNBC trials. Methods: ER, PgR, and HER2 status was determined by central review of tumors collected from three phase III randomized trials: GEICAM 9906 (n=820), NCIC CTG MA.5 (n=476) and MA.12 (n=398). PAM50 intrinsic subtyping (BLBC, HER2-enriched, Luminal A, Luminal B and Normal-like) was performed using the qRT-PCR-based assay. Quantitative ER/PgR expression by IHC and subtype was tested using ANOVA and Fisher’s exact test. Results: Of 1,694 tumors, 15% were BLBC, 21% HER2-Enriched, 33% Luminal A, 25% Luminal B and 4% Normal-like. B...