RESUMO
Primary osteosarcoma of the breast is an extremely rare and aggressive tumor representing 12.5% of mammary sarcomas. It is associated with early recurrence and hematogenous rather than lymphatic spread, most commonly to the lungs. We report a case of 55 year old lady with gradually increasing painless lump measuring 7 × 6 cm in upper outer quadrant of left breast of 2 months duration. FNAC smears revealed pleomorphic spindle cells admixed with osteoid and chondroid material. A differential diagnosis of metaplastic carcinoma and malignant phyllodes tumor was offered. Histopathology of the modified radical mastectomy specimen showed predominantly osteosarcomatous areas. Epithelial component was not seen even after extensive sampling of the tumor. On immunohistochemistry, vimentin, S100 and CD68 were positive while AE1/AE3, CAM5.2, ER, PR, Her2-neu and desmin were negative confirming a diagnosis of osteosarcoma. Primary osteosarcoma of breast must be differentiated from metaplastic carcinoma and malignant phyllodes tumor because of different biological behaviour and treatment approach. Immunohistochemistry plays a pivotal role in the diagnosis of this tumor.