Myositis ossificans mimicking metaplastic breast cancer on core needle biopsy

Summary Myositis ossificans (MO) is an uncommon myofibroblastic proliferation that may closely mimic sarcoma. A 33-year old woman presented with a 2 cm breast mass. A core needle biopsy showed highly cellular spindle cell proliferations with atypia and high proliferation activity. Focal areas of immature osteoid-like matrix were seen. After immunohistochemical analysis, a diagnosis of metaplastic breast cancer was made. Due to the proximity of the tumor to the rib, resection of the tumor including partial resection of the rib was carried out. Complete examination of the excised lesion allowed the correct diagnosis of MO. Only the resection specimens contained a predominance of mature organoid bone tissue admixed with variably cellular myofibroblastic proliferations with the typical zonal pattern, characteristic of MO. The diagnosis of MO was not possible on the initially performed core needle biopsy since biopsy material reflected only the central proliferative portion of the lesion showing increased pleomorphism and early osteoid formation. This case highlights the pitfall related to this unusual presentation of a benign lesion and points to the necessity to include MO in the differential diagnosis of triple negative metaplastic breast cancer.