Fasciitis ossificans of the breast

We report a rare case of fasciitis ossificans of the breast. A nodule of the breast was incidentally found in a 77-year-old woman without a history of trauma. The painless lesion was located 3 cm from the nipple in the lower outer quadrant of the left breast. Excisional biopsy was performed, and there has been no evidence of recurrence or metastasis for 2 years. The 1.8 cm diameter nodule was well demarcated from the adjacent tissue and located 2.5 cm beneath the skin. Histologically, the lesion was composed of uniform woven bone trabeculae with rimming of osteoblasts and fibrous stroma. At the periphery, spindle cells actively proliferated in edematous stroma, demonstrating uniform nuclei without increased chromatin, pleomorphism, or evident nucleoli. We counted 2 mitotic figures per 10 high-power fields, but no atypical forms were observed. Spindle cells were immunoreactive for vimentin and alpha-smooth muscle actin, suggesting myofibroblastic differentiation. Fasciitis ossificans is histologically identical to myositis ossificans, but tends to present no zonation phenomenon. We considered this lesion as fasciitis ossificans since it was situated at the superficial layer of the mammary gland. To avoid an unnecessarily aggressive treatment, fasciitis ossificans, a benign bone-forming nodule, needs to be considered in the differential diagnosis of breast hard tumor.