Objective: Our study investigated the morphological and immunohistochemical characteristics of invasive breast carcinoma of no special type (IBC-NST) with medullary pattern to explore the inconsistencies between the structural and clinical traits of this category of tumor.

Material and Method: The breast carcinoma samples (n = 26) with medullary pattern (defined according to established criteria) were subjected to immunohistochemical assays of the following receptors: ER, PR, HER2/neu, Ki-67, p53, Bcl-2, VEGF, MMP1, E-cadherin, EGFR, Hsp70, Hsp90, CD20, CD3, CD4, CD8, CD68, CD163, CD56, CD138, MPO, S100, IgG, IgM, and PD-L1.

Results: IBC-NST with medullary pattern was found to have negative expression of ER, PR, and HER2/neu; strong positive expression of Kі-67, mutant р53, Bcl-2, E-cadherin, EGFR, and PD-L1; moderate positive expression of Hsp70 and Hsp90; and low or negative expression of VEGF and MMP1. Furthermore, there was pronounced variability in the qualitative composition of tumor immune infiltrates with regards to T-lymphocytes, B-lymphocytes, macrophages, plasmocytes, and granulocytes.

Conclusion: IBC-NST with medullary pattern has many unfavourable morphological and immunohistochemical prognostic characteristics, which are balanced against the pronounced protective properties of the tumor cells and the qualitative characteristics of the tumor microenvironment. These can lead to a favourable disease course despite the relatively adverse features of the carcinoma cells.