Large Solitary Lung Metastasis of a Matrix-Producing Metaplastic Breast Carcinoma: A Diagnostic Challenge.

Metaplastic breast carcinoma represents a diverse category of invasive breast cancers distinguished by the transformation of neoplastic epithelial cells into squamous cells or cells with mesenchymal appearance. Matrix-producing breast carcinoma is a variant of metaplastic breast carcinoma, an exceedingly uncommon malignancy accounting for less than 1% of all breast tumors. The precise origin of this tumor remains elusive; some molecular research points to a potential derivation from myoepithelial cells, while other studies emphasize the possibility of neoplastic transformation originating from multipotent stem cells. We report a case of recurrent matrix-producing breast carcinoma. The patient presented with a breast mass. The tumor cells displayed a lack of reactivity for estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2), and exhibited a Ki-67 proliferation index of approximately 40%. Additionally, the tumor cells demonstrated significant reactivity for cytokeratins and S100. The patient underwent surgery, radiation, and chemotherapy and then developed metastasis to the lower lobe of her left lung, seven years after primary diagnosis. Diagnosis of metastasis was confirmed by comparing the metastasis to the primary tumor and staining with a panel of immunohistochemical stains. The patient is currently undergoing chemotherapy and immunotherapy.