RESUMEN
Phyllodes tumors account for less than 1% of tumors of the mammary gland, have both epithelial and stromal components and are classified as benign, borderline and malignant. The malignant tumors are highly heterogeneous: they can differentiate to liposarcomas, fibrosarcomas, rhabdomyosarcomas, chondrosarcomas or osteosarcomas. The differentiation to osteosarcoma is extremely rare, constitutes 1.3% of cases and is very aggressive. The standard treatment of these tumors is surgical. The role of radiotherapy and chemotherapy is not clear. However, in patients in whom wide surgical margins are not achieved, adjuvant radiotherapy can be of help. We report a 63 years old female with a right breast osteosarcoma with an osteoclastic component, originating in a phyllodes tumor. The tumor was excised surgically and afterwards she was treated with 10 sessions of 50 Gy of radiotherapy in 25 fractions. She has remained free of disease for the last 10 months.