Characterization and spatial distribution of infiltrating lymphocytes in medullary, and lymphocyte-predominant triple negative breast cancers

Medullary carcinoma of the breast (MedBC) is a rare histological type that accounts for less than 5% of all invasive breast cancers. Here, we performed an exploratory study aimed to determine whether imaging mass cytometry (IMC) can be used to characterize the immune infiltration and the spatial distribution heterogeneity in the rare subtype of MedBC compared to atypical MedBC and TNBC-TILS⁺ tumors. In both MedBC and TNBC-TILs⁺, there was a notable enrichment of immune cells in the peripheral regions of the tumors, whereas in atypical MedBC, the immune cells exhibited a central enrichment pattern. This distribution of infiltrated cells reflects an active immune recruitment correlated to more favorable prognosis. In MedBC, spatial analysis shows that immune cells are localized at a greater distance from the tumor cells. IMC highlights the heterogeneity of immune microenvironment across three main subtypes of breast tumors and could help to define distinct immune patterns.