Synchronous Metaplastic Breast Carcinoma with Mesenchymal Differentiation and Triple-Negative Invasive Papillary Carcinoma: A Case Report and Literature Review

Background: Metaplastic breast carcinoma with mesenchymal differentiation is a rare and morphologically heterogeneous malignancy that may show a biphasic epithelial and sarcomatoid pattern historically termed “carcinosarcoma.” Its synchronous occurrence with invasive papillary carcinoma is exceptionally uncommon and may create diagnostic and therapeutic difficulties. Methods: We report the clinicopathological course of a premenopausal woman with two synchronous breast malignancies. Clinical, radiological, histopathological, and immunohistochemical findings were reviewed, including PD-L1 assessment using the 22C3 antibody, and interpreted in the context of the relevant literature. Results: Definitive surgical pathology demonstrated a 125-mm metaplastic breast carcinoma with mesenchymal differentiation showing carcinosarcoma morphology and a separate 22-mm high-grade triple-negative invasive papillary carcinoma. Sarcomatoid predominance led to the initial interpretation of malignant phyllodes tumor on core biopsy. Axillary lymph-node metastases showed papillary morphology and a concordant immunophenotype, strongly supporting their relationship to the invasive papillary carcinoma. Later supraclavicular recurrence and pulmonary metastases could not be conclusively attributed to either primary tumor. PD-L1 combined positive scores were 25 in both primary tumors and 35 in the supraclavicular metastasis. Pembrolizumab-based therapy was followed by radiological regression and clinical stability. Conclusions: This case highlights the importance of representative sampling, careful characterization of synchronous tumors, comparison of metastatic deposits with all primary lesions, and biomarker reassessment during progression. The observed response should be interpreted in the context of PD-L1-positive metastatic triple-negative breast cancer rather than as evidence of histology-specific pembrolizumab efficacy.