[Case #1. Phyllodes tumors grading: Practical insight and recommendations].

Phyllodes tumors are rare breast tumors (0.3 to 1% of all breast tumors). They are fibro-epithelial neoplasms that can be benign, borderline, or malignant. The majority of phyllodes tumors are benign (60 to 75%). The diagnostic criteria for malignant phyllodes tumours are well defined in the 2019 WHO classification. Malignant phyllodes tumours are diagnosed when malignant heterologous elements are present (excluding well-differentiated liposarcoma) or when all of the five following features are present: marked nuclear pleomorphism, stromal overgrowth at 40× magnification (objective 4× and eyepiece 10× magnification), increased mitotic index (≥5 mitoses/mm; ≥10 mitoses/10 HPF), diffuse hypercellularity, and infiltrative borders. Distant metastases appear within five to eight years after the diagnosis of phyllodes tumour and occur in 16% of malignant phyllodes tumours. At the metastatic site, the stromal component is typically observed, with common metastatic locations being the lungs, bones, and heart. Strict adherence to the grading criteria of the 2019 WHO classification does not reliably predict the risk of metastatic recurrence, especially regarding the requirement for the simultaneous presence of all five adverse criteria. Several recent studies have challenged this notion, proposing new diagnostic criteria that, however, require validation in prospective studies before being implemented.