Malignant phyllodes tumour of the breast mimicking endometriosis

A 63-year-old woman presented with a clinically malignant mass. Core biopsy showed features resembling endometriosis. The glands were GATA3 and oestrogen receptor positive consistent with mammary origin and had no myoepithelial layer. The excision also showed a fibroepithelial component with stromal overgrowth, frequent mitoses and invasive margin consistent with a malignant phyllodes tumour. KMT2D and SETD2 mutations were present in both the conventional phyllodes tumour and endometriosis-like areas and are also described in endometriosis raising interesting questions about these lesions. This unusual pattern is a potential diagnostic pitfall, so it is helpful to be aware of it.