Malignant Phyllodes Tumour of the Breast with Liposarcomatous Differentiation: A Rare Case Report

Background: Phyllodes tumours (PTs) are rare fibroepithelial neoplasms of the breast, constituting less than 1% of all breast tumours. Among these, malignant PTs with heterologous sarcomatous differentiation, particularly liposarcomatous transformation, are exceedingly uncommon and pose significant diagnostic challenges. Case Presentation: We report the case of a 55-year-old woman who presented with a painless, mobile breast lump initially suspected to be a benign lesion. Imaging suggested a lipoma, but fine-needle aspiration cytology revealed atypical cells, prompting surgical excision. Histopathological examination confirmed a malignant phyllodes tumour with liposarcomatous differentiation. Immunohistochemistry showed S100 positivity and was negative for MDM2 and CDK4, supporting the diagnosis and excluding primary breast liposarcoma. The patient subsequently underwent a left mastectomy, with no residual tumour detected and remains under close follow-up without recurrence. Conclusion: This case underscores the importance of integrating clinical, radiological, histopathological, and immunohistochemical findings in diagnosing rare variants of malignant PTs. Recognizing liposarcomatous differentiation within PTs is critical, as it influences prognosis and helps differentiate them from primary breast sarcomas.