Malignant Phyllodes Tumor on a Mastectomy Scar with Benign Cartilaginous Differentiation: A Diagnostic Challenge

Phyllodes tumors are rare fibroepithelial neoplasms of the breast, accounting for less than 1% of all breast tumors. Among them, malignant phyllodes tumors (MPTs) represent the most aggressive form and may, in rare cases, exhibit heterologous differentiation. We report an exceptional case of MPT arising on a mastectomy scar in a patient previously treated for an invasive ductal carcinoma, with no history of radiotherapy. The chest wall mass, fixed and voluminous, was initially investigated through two biopsies: the first was non-contributive, while the second revealed benign cartilaginous metaplasia with no signs of malignancy. This interpretation was later contradicted by a third, deeper biopsy, which revealed a hypercellular stromal proliferation with marked atypia and a Ki-67 index of 30%, consistent with MPT. No cartilaginous tissue was identified in this third sample. This rare case illustrates the diagnostic challenges posed by MPTs with unusual differentiation, particularly when a benign heterologous component is revealed before the malignant stroma is detected. It emphasizes the importance of thorough clinicopathological correlation and repeated sampling in cases of diagnostic discordance. The tumor was deemed unresectable, and palliative chemotherapy with anthracyclines followed by taxanes led to partial symptom control. To our knowledge, this is the first documented case of an MPT with benign cartilaginous differentiation arising on a mastectomy scar, without prior radiotherapy. This observation raises significant diagnostic and therapeutic considerations and underscores the need for long-term surveillance of surgical sites in breast cancer patients.