Agreement in Risk Prediction Between the 21‐Gene Recurrence Score Assay (Onco type DX®) and the PAM50 Breast Cancer Intrinsic Classifier™ in Early‐Stage Estrogen Receptor–Positive Breast Cancer

Purpose. To compare risk assignment by PAM50 Breast Cancer Intrinsic Classifier™ and Onco type DX_Recurrence Score (RS) in the same population. Methods. RNA was extracted from 151 estrogen receptor (ER) + stage I–II breast cancers and gene expression profiled using PAM50 “intrinsic” subtyping test. Results. One hundred eight cases had complete molecular information; 103 (95%) were classified as luminal A ( n = 76) or luminal B ( n = 27). Ninety‐two percent ( n = 98) had a low ( n = 59) or intermediate ( n = 39) RS. Among luminal A cancers, 70% had low ( n = 53) and the remainder ( n = 23) had an intermediate RS. Among luminal B cancers, nine were high (33%) and 13 were intermediate (48%) by the RS. Almost all cancers with a high RS were classified as luminal B (90%, n = 9). One high RS cancer was identified as basal‐like and had low ER/ ESR1 and low human epidermal growth factor receptor 2 (HER2) expression by quantitative polymerase chain reaction in both assays. The majority of low RS cases were luminal A (83%, n = 53). Importantly, half of the intermediate RS cancers were re‐categorized as low risk luminal A subtype by PAM50. Conclusion. There is good agreement between the two assays for high (i.e., luminal B or RS > 31) and low (i.e., luminal B or RS < 18) prognostic risk assignment but PAM50 assigns more patients to the low risk category. About half of the intermediate RS group was reclassified as luminal A by PAM50.