Molecular Subtyping of Invasive Breast Carcinoma by Immunohistochemistry and Five-Year Survival Study

Background: Global gene expression profiling for Invasive breast carcinoma (IBC) has identified intrinsic subtypes of IBC with differing clinical outcomes and response to therapy. As genotyping assays are limited by availability and cost, we have used Immunohistochemistry (IHC) surrogates to classify IBC into molecular subtypes. Methods: Representative tumor blocks of 158 surgical specimens of IBC between 2007 to 2017, were selected and IHC done for ER, PR, Her2, Ki67, CK 5/6 and EGFR. The cases were classified into 7 Molecular subtypes (Luminal A, Luminal B, Luminal HER2PR+, Luminal HER2PR-, HER2Enriched, Basal like (BLBC) and non classifiable (NCBC) and correlated with clinico-pathological findings. Five- year survival rate was calculated for patients diagnosed between 2007 to 2013. Result: The most common subtype was Luminal A (31.0%), followed by Luminal B (25.3%), NCBC (14.6%) and HER2 enriched (13.3%). Among post-menopausal women, common subtypes were Luminal A (33.8%) and Luminal B (24.4%). Among premenopausal women, most cases were NCBC (27.8%) and BLBC (22.2%).  61.2% of Luminal A were Grade2 and 22.4% were Grade 1. Many cases of Luminal B and HER2 positive cases were of Grade3 (45.0%) and (57.1%) respectively. Of the triple-negative category (BLBC & NCBC), 73% were Grade3 with statistically significant correlation (p value < 0.001). Most of these cases were in Tumor stage T2 (70%), followed by T1 (22.3%). Nodal metastasis was seen in 39.6% and 65% respectively of Luminal A and Luminal B subtypes. Distant metastases on follow-up were present in 15.8%, which included HER2 enriched subtype (28.5%), followed by BLBC (20.0%) and NCBC (17.4%). Luminal A cases, had better survival accounting for 88% of all survivors. Conclusion: Molecular subtyping of IBC using IHC was useful to understand the clinicopathological distinctiveness of each subtype.