myofibroblastoma

my·o·fi·bro·blas·to·ma

(mī'ō-fī'brō-blas-tō'mă), a benign breast tumor composed of myofibroblasts.

myofibroblastoma

(1) A well-circumscribed mass, typically seen in the breast, that entraps fat and has variable chrondoid metaplasia and stroma similar to that of spindle cell lipoma. 
(2) An older term for what is sometimes regarded as a morphologic variant of fibromatosis.