Case: A woman in her 30s presented with a rapidly enlarging right-breast mass (33 × 31 × 28 mm). Core biopsy showed a high-grade sarcomatoid malignancy with frequent mitoses and necrosis. After neoadjuvant chemotherapy, mastectomy revealed grade 3 poorly differentiated sarcomatoid carcinoma with a sheet-like growth pattern with patch necrosis. The tumour cells were characterised by spindled to epithelioid morphology, eosinophilic cytoplasm, brisk mitotic activity (46/10HPF) and pleomorphic nuclei, including multinucleated forms and frequent bizarre nuclei. Nodes were negative. Tumour cells were positive for TRPS1, SOX10, S100, PRAME and p53, with scattered p63 and rare pancytokeratin. They were negative for GATA3, CK5/6, CK14, CK7, SMA/desmin, ERG, D2-40, melanocytic markers (HMB45, MITF, Melan-A) and ER/PR/HER2. Ki-67 was ∼90%. A melanoma panel (BRAF, KIT, NRAS) identified no mutations.
Song et al. (Sun,) studied this question.