Primary breast osteosarcoma is a type of breast tumor with bone producing osteoid matrix [6]. It is not only a very rare breast malignancy but also an unusual location for an extraskeletal sarcoma [7]. Although rare, its diagnosis is important as the imaging features, treatment and prognosis differ from other breast cancers. As with all sarcomas, it is defined by the underlying type of tissue with the most common subtype including fibrohistiosarcoma, myxofibrosarcoma and angiosarcoma [2].

Primary breast osteosarcoma has to be differentiated from metaplastic carcinoma and cystosarcoma phyllodes, that can both present with similar imaging features [8]. The clinical presentation is a breast mass, with rapid growth but rarely associated with pain [9]. Nodal involvement is uncommon, but metastases are frequent. Lung is the most commonly organ involved, followed by bone. The survival is poor, with an overall 5-year survival rate of 38% [10]. On mammography, the primary lesion is usually a large, dense, calcified mass [11]. However, in some cases, calcifications can be absent [12]. Ultrasound is helpful for lesion characterization, staging and biopsy guidance and usually shows important acoustic shadowing limiting analysis [2]. MRI is useful to assess the extent of the mass within the breast, to search out for other masses, assess for chest wall invasion usually showing a high T2 signal mass with enhancement [1]. Whole-body contrast-enhanced CT scan or 18FDG PET-CT are of interest to detect metastatic extension [6]. The final diagnosis of primary breast osteosarcoma is made on pathology, demonstrating osteosarcomatous matrix and showing that the lesion does not arise from adjacent bone (sternum or ribs) [13]. Immunohistochemistry is helpful to establish the diagnosis, with the use of epithelial markers such as CKAE1/AE3, CK5/6, EMA, SMA, CK7, vimentin, MAC-387 and others [14]. Excisional biopsy (complete tumorectomy), as this was done in our patient, is preferred to fine needle biopsy, as the mass can be very firm due to the presence of calcifications that can lead to inconclusive results as in our case [6]. As with any subtype of sarcoma, the treatment is the complete surgical excision with large margins [11]. Local recurrence rates were reported as high as 67% after tumorectomy and 11% after mastectomy [5, 15]. If no local node involvement is seen, axillary node dissection is not recommended, since this type of sarcoma does not spread via lymphatic route [16]. In this regard, our case was unusual as a pathologic intra-mammary lymph node was found. There is no consensus about the administration of adjuvant chemotherapy. However, it is generally recommended as it evidenced to improve survival rates, but no standard dose has been established yet [10]. For more advanced cases with metastasis, chemotherapy is the main treatment. Drugs used are the same as with other sarcomas and include cisplatin, doxorubicin, ifosfamide and methotrexate [17].

Acknowledgments: The authors declare no conflict of interest.