Diagnosis:  fibromatosis of the breast. 

Immunohistochemistry:

• CD34 negative
• desmin negative
• vimentin positive
• Ki-67 positive 0%
• B-catenin positive
• S-100 negative

Fibromatosis or desmoid tumor of the breast is a rare tumor and represents <2% of all breast tumors. This tumor is a proliferation of fibroblasts or myofibroblasts that infiltrate the stroma or breast fat and are low grade, but they are very aggressive and affect women young and old.

The etiology is idiopathic but could be associated with Gardner syndrome and familial adenomatous polyposis syndrome. According to some authors, it is secondary to trauma or surgery.

This tumor should be treated with wide excision.