Cross-sectional study of immunochemistry versus molecular breast cancer subtyping among African populations

Breast cancer subtyping based on immunohistochemistry (IHC) guides systemic therapy where gene expression profiling assays are unavailable, yet its validity against molecular reference standards remains poorly characterized in African populations, where laboratory infrastructure and access to pathology services are limited. A cross-sectional study evaluated concordance between IHC-based breast cancer subtyping and PAM50 intrinsic subtypes in East and West African populations. In this study of 460 women with breast cancer in Kenya, externally validated in 451 women with breast cancer in Ghana, estrogen receptor, progesterone receptor, and HER2 status correctly classified most basal-like tumors but misclassified most PAM50 luminal B tumors as luminal A. Adding Ki-67 or histological grade increased luminal B classification among IHC-defined luminal tumors. The authors conclude that these findings suggest that incorporating Ki-67 or histological grade may improve identification of luminal B disease when gene-expression assays are inaccessible.