We describe a case of primary osteosarcoma of the breast in a 56-year-old woman. She presented with a 3-month history of a palpable, painless right breast mass without axillary lymphadenopathy. Neither past radiotherapy nor family history of breast cancer was found. She received a right modifi ed radical mastectomy under the diagnosis of breast cancer. Grossly, the specimen mass was not attached to the underlying ribs or the sternum. Histopathologically, the sections of tumor revealed a high-grade osteoblastic osteosarcoma with characteristic neoplastic osteoid formation. Postoperative radiation therapy was performed subsequently. Six years later, she experienced local recurrence and soon suffered from metastases to lung and bone. The sections of recurrent and metastatic tumors showed morphology identical to the original breast tumor.
Finally, she died 75 months after the initial diagnosis. Primary mammary osteosarcoma is an extremely rare tumor. The differential diagnoses for primary osteosarcoma of the breast based on the histopathology and immunohistochemistry as well as the clinical features are demonstrated.