Financial support: none suMMary Primary osteosarcoma of the breast (POB) is an extremely rare and aggressive tumor. Differential diagnosis of POB includes osteosarcoma of the chest wall and metaplastic breast carcinoma. Imaging tests that exclude the existence of a direct connection between the tumor and chest wall, as well as histopathological and immunohistochemical studies that rule out the presence of an epithelial component are required for the diagnosis of POB. We report a case of a 69-year old woman with POB. Imaging and pathological findings are presented. Therapeutic approach is discussed in the light of current knowledge, including potential complications.Keywords: breast, breast neoplasms, sarcoma, immunochemistry.A 69-year-old woman presented with a 6-month history of a palpable, painless mass in her left breast. Clinical examination revealed a hard, mobile, well-circumscribed, 10 cm mass, occupying practically the entire left breast. Contralateral breast, axilla or nipples showed no abnormalities. She denied having any history of breast trauma, radiotherapy, or breast cancer.Mammography revealed a relatively well-defined, hyperdense irregular mass without calcifications, unconnected to the underlying sternum and ribs ( Figure 1A). Ultrasound revealed a hypoechoic irregular mass with hyperechoic areas, indistinct margins, and posterior acoustic shadowing ( Figure 1B). Core needle biopsy was performed and microscopy showed a malignant poorly differentiated mesenchymal tumor.Thoracic and abdominal computed tomography showed no distant metastasis. The patient underwent simple mastectomy and sentinel lymph node biopsy. Grossly, the cut surface of the tumor was white, hard, with a stony consistency in the center and well-defined margins. The tumor measured 10x9x7 cm (Figure 2).Microscopy showed a malignant immature mesenchymal bone-forming tumor. There were foci of necrosis and cartilage, containing osteoid matrix, some of which were irregularly calcified, surrounded by atypical spindle cells and osteoclast-like multinucleated giant cells. Tumor mitotic count was 16 mitoses/10 high-power field. There was no skin and lymphovascular invasion. Sentinel lymph node was tumor-free. On immunohistochemistry, tumor cells were positive for vimentin. CD68 (KP1) and alpha-1-antitrypsin were positive in osteoclast-like giant cells. Cytokeratin (AE1/AE3), S-100, epithelial membrane antigen (EMA), p63, epidermal growth factor receptor (EGFR), CD99, estrogen and progesterone receptors, and HER2 were negative. The Ki-67 index was 5%. Extensive tumor sampling failed to identify any tumor that was biphasic or had epithelial component. A highgrade primary osteosarcoma of the breast (POB) was di-
