Medullary carcinoma shows a diffuse growth pattern with no glandular differentiation or intraductal component. The tumor cells have high-grade nuclei and abundant eosinophilic cytoplasm. They are arranged in broad sheets, nests, or anastomosing trabeculae. The cytoplasmic borders are indistinct creating a syncytial appearance. A characteristic feature is the presence of a prominent lympho-plasmacytic infiltrate within and around the tumor.