Myofibroblastoma is a rare mesenchymal benign tumor that occurs in various tissues. We report a case of myofibroblastoma in the breast of an elderly man, focusing on ultrasound findings and histopathology.The patient was a man in his 80s. A left breast tumor was incidentally detected by computed tomography（CT）.Ultrasound examination revealed a smooth, oval-shaped mass measuring 27×27×15 mm in the upper left breast. Color Doppler imaging showed blood flow at the margins of the tumor. Elastography revealed that elasticity was maintained, with a Tsukuba elasticity score of 2. The tumor was homogeneously and strongly enhanced on contrast-enhanced magnetic resonance imaging（MRI）.Core needle biopsy revealed proliferation of spindle-shaped cells; thus myofibroblastoma was suspected and tumor resection was performed. Macroscopically, it was a 25×22×18 mm, elastic, soft, oval tumor with clear margins. The cut surface was light brown and showed numerous fused nodules. Histologically, spindle-shaped cells proliferated in bundles and small portions of adipose tissue were observed. No mammary glands were observed. Immunohistochemical staining was positive for desmin, which indicated differentiation into smooth muscle cells, and negative for S100, SMA, and CD31. Thus, we diagnosed it as myofibroblastoma. The patient is undergoing follow-up and no recurrence has been seen for two years after surgery. Myofibroblastoma may be included as a differential diagnosis when a well-circumscribed, oval mass with a heterogeneous internal echo is observed.

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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