Breast, borderline phyllodes tumour

Fibroepithelial lesions are tumours of the specialized intralobular stroma of the breast, and those with leaf-like growth and stromal cellularity are classified as phyllodes tumours. Phyllodes tumours are classified as benign, borderline, and malignant in the WHO system; however, the diagnosis is based on multiple features. In this case, there is prominent stromal overgrowth. Other features are less concerning, including atypia, mitotic activity, and tumour borders. Therefore, this tumour was classified as a borderline phyllodes tumour.