Abstract
Myofibroblastic dermatofibroma (MFD) is an unusual neoplasm characterized by a predominance of myofibroblastic differentiation. It is extremely rare and it is not well described. Expressions of smooth muscle actin, CD34, S-100, desmin, CD31, and Factor XIIIa were evaluated along with hematoxylin-eosin and trichrome staining of fixed tissue specimens from a fibrohistiocytic neoplasm. The neoplasm demonstrated a storiform pattern characteristic of fibrohistiocytic origin. It was strongly and diffusely positive for smooth muscle actin and vimentin, and negative for all other stains performed. A trichrome stain showed the entire tumor to be blue, demonstrating the collagenous and fibrous tissue to a marked degree. MFD is a distinct variant of dermatofibroma characterized by a predominance of myofibroblastic differentiation.