Objective: Presentation of variable morphology of the so-called matrix-producing carcinoma, the rare variant of metaplastic breast carcinoma. Establish the extent of myoepithelial phenotype by immunohistochemistry and assess behaviour of the lesion. Design: Review of clinicopathologic data of 11 cases. Setting: Private Biopsy Lab s.r.o., Pilsen and Šikl’s Department of Pathology, Charles University and Faculty Hospital, Pilsen. Methods: Included are eleven cases from Breast Registry, Biopsy Lab s.r.o., Pilsen and Pathology Department, Faculty Hospital in Hradec Králové and Martin and from Biopsy Lab in Hořovice. Analyzed were morphology, stage, grade and follow up. Myoepithelial phenotype was demonstrated immunohistochemically by LSAB+ system (DAKO) and seven conventional myoepithelial markers (GFAP, aktin S, calponin, cytokeratin 14, p63, CD10 and P-cadherin) were used. Oncogene p53 and Her2/neu were also detected. Results: Matrix-producing carcinoma is an extremely rare type of metaplastic carcinoma of the breast, where high-grade epithelial component continually merges with heterologous mesenchymal chondroid component without overt spindle cell sarcomatoid pattern in between. There were all women at the age of 32 to 86-years (average 54 years, median 52 years) and the maximum tumor diameter ranged from 18 to 60 mm (average 32 mm). The axillary LN was positive in six cases in the time of diagnosis. The follow up available in 9 women ranged from three months to ten years (average 24 months); dissemination of the disease was observed in three cases; 2 of these patients died of disease after three and ten years, respectively. The heterologous mesenchymal chondroid component was produced in two different ways; one displayed typical structure of low-grade hyaline cartilage, in the second one the epithelial tumorous cells were embedded in the homogenous eosinophilic extracellular matrix giving appearance of chondroid aura. Metaplastic component constituted 5-75% of the tumor volume. Immunohistochemical evidence of myoepithelial differentiation in all neoplasms was demonstrated, with at least two conventional myoepithelial markers (actin S, calponin, GFAP, CK14, CD10, p63 and P-cadherin) being positive in every case. Expression of p53 was identifi ed in six cases, all tumors were Her2/neu negative. Histologically the metastases were formed either by carcinoma cells only, or more frequently, they replicated the structure of primary lesion. Conclusion: As a rare entity, matrix-producing carcinoma of breast displaying myoepithelial phenotype, deserves separate position in tumor classifi cation. It differs from conventional myoepithelial carcinoma and from heterologous metaplastic carcinoma, where the matrix emanate from undifferentiated sarcomatous tissue, but precise histogenesis in not clear yet. This is a very aggressive lesion and our fi ndings show that previous reports of indolent behaviour were preliminary. Spectrum of differential diagnosis, namely benign mixed tumor, phylloid tumor, primary breast sarcoma and colloid carcinoma warrant clear knowledge about this unique entity.

        Key words: breast, metaplastic carcinoma, matrix-producing carcinoma, sarcomatoid carcinoma, heterologous, myoepithelial differentiation