Malignant Phyllodes Tumor of the Breast With Multiple Heterologous Sarcomatous Differentiations and Focal Rhabdoid-Like Features: A Case Report.

Malignant phyllodes tumors are rare fibroepithelial breast neoplasms characterized by malignant stromal proliferation and variable biological behavior. We report an exceptionally rare case of a malignant phyllodes tumor with multiple heterologous sarcomatous differentiations and focal rhabdoid-like features. A 50-year-old woman presented with a rapidly enlarging right breast mass. Imaging demonstrated a large Breast Imaging Reporting and Data System (BI-RADS) 4 lesion infiltrating the adjacent pectoral muscle without definite distant metastases. Core needle biopsy revealed an undifferentiated malignant spindle-cell proliferation with diffuse desmin and smooth muscle actin expression, raising suspicion for primary breast leiomyosarcoma or malignant phyllodes tumor with myoid differentiation. The patient underwent right mastectomy with axillary lymph node dissection. Histopathological examination confirmed a 9.5-cm grade 3 malignant phyllodes tumor with stromal overgrowth, high mitotic activity, heterologous chondrosarcomatous, liposarcomatous, and myoid differentiation, and focal rhabdoid-like features without myogenin expression. Surgical margins and lymph nodes were negative. Adjuvant radiotherapy (50 Gy in 25 fractions) was administered. After 17 months of follow-up, no local recurrence or distant metastasis was observed. This case highlights the diagnostic challenges posed by these rare tumors and emphasizes the importance of extensive histopathological sampling, comprehensive immunohistochemical evaluation, complete surgical excision, and selective use of adjuvant radiotherapy in high-risk patients.