Preoperative diagnosis of a giant cell tumor of soft tissue arising from the breast by ultrasound-guided core needle biopsy.

Giant cell tumors of soft tissue (GCT-ST) arising in the breast are extremely rare. Herein, we report a case of a 45-year-old woman with a 5-cm mass in her left breast. Ultrasonography revealed a mainly well-circumscribed mass that contained a cystic lesion. Magnetic resonance imaging showed a fibrous capsule-covered mass that contained a high-intensity area, suggesting hemorrhaging. Ultrasound-guided core needle biopsy (CNB) revealed mononuclear histiocytic cells with a round shape or spindled appearance that was mixed with multinucleated giant cells. Immunohistochemical analysis revealed CD68-positive staining in the mononuclear and giant cells but negative staining for pancytokeratin. Preoperatively, the tumor was highly suspected of being GCT-ST. Histopathological results after a left mastectomy showed similar findings to CNB. The final diagnosis was GCT-ST in the breast. To the best of our knowledge, this is the first case report of a GCT-ST arising in the breast diagnosed by ultrasound-guided CNB.