Breast, Malignant phyllodes tumour, H&E stain

Malignant phyllodes tumours are fibroepithelial neoplasms with malignant stromal growth. Criteria for malignancy include stromal overgrowth, stromal hypercellularity, stromal cell atypia, mitotic activity, and invasive tumour borders. In this case, even though stromal overgrowth and heterologous elements are not seen, there is marked pleomorphic atypia and marked cellularity, along with increased mitotic activity (at least 9 per 10 high power fields).