[A Case of Malignant Phyllodes Tumor Difficult to Distinguish from Spindle Cell Carcinoma]

A 70-year-old woman noticed a mass in her right breast. Breast ultrasonography showed a low echo mass with a smooth border to the greatest dimension measuring 5.4 cm. Needle biopsy showed an increase in the number of spindle-shaped cells with diffuse fascicles having nuclei with seasonal polymorphisms in the stroma, which led to the final diagnosis of spindle cell sarcoma. Immunohistochemistry analysis showed a CAM5.2-negative, AE1/AE3 partly-positive, bcl-2-positive, ER-negative, PgR-negative, SMA-positive, S-100-negative, desmin-negative, CD34-negative, and keratin 5/6-negative tumor that was suspected to be a phyllodes tumor. Differential diagnoses included sarcoma-likecance r, stromal sarcoma, and malignant phyllodes tumor. Breast mass resection was performed for a definitive diagnosis. The final pathological analysis showed a malignant phyllodes tumor. To date, we have encountered 1 case of malignant phyllodes tumor that was difficult to distinguish from a spindle cell sarcoma. Excisional biopsy is required for a definitive diagnosis.