214 Metaplastic Breast Carcinoma with Spindle Cell and Squamous Differentiation Arising in a Phyllodes Tumor: A Rare and Complex Case

Introduction/Objective Metaplastic breast carcinoma (MBC) is a rare subtype of triple-negative breast cancer with both epithelial and mesenchymal features, comprising less than 1% of all breast malignancies. Even rarer is the occurrence of MBC arising within a phyllodes tumor and presents significant diagnostic and therapeutic challenge. Methods/Case Report A 72-year-old female with a history of kidney and thyroid malignancies presented with right breast mass. Imaging revealed a 4.0x3.3x2.7 cm heterogeneously enhancing lesion in the upper central right breast. Ultrasound-guided core needle biopsy demonstrated a triple-negative, poorly differentiated ductal carcinoma with adenosquamous and spindle cell features, consistent with metaplastic carcinoma. The patient received neoadjuvant therapy, followed by right mastectomy. Histopathologic examination confirmed high-grade metaplastic carcinoma arising within a phyllodes tumor, with both spindle cell and squamous differentiation. Immunohistochemistry demonstrated strong positivity for p63, p40, CK5/6, and OSCAR in malignant epithelial/squamous components. GATA3 confirmed breast origin, S100 was focally positive. The spindle cell component expressed vimentin and BCL2. Ki-67 was elevated (30%) in the metaplastic components. Results NA Conclusion This rare case illustrates a highly unusual presentation of metaplastic breast carcinoma arising within a phyllodes tumor, underscoring the need for thorough histopathological and immunohistochemical evaluation to guide appropriate management. As such cases remain exceptionally rare, further studies are needed to clarify their behavior, prognosis, and treatment strategies.