Fibrosarcomatous dermatofibrosarcoma protuberans with myofibroblastic differentiaion: a histologically distinctive variant [corrected].

We report two examples of dermatofibrosarcoma protuberans with fibrosarcomatous change, in which the fibrosarcomatous component of the tumor demonstrated focal myofibroblastic differentiation. This was recognized by light microscopy as plump myoid cells associated with abundant extracellular collagen production, immunohistochemically by positive staining for smooth muscle and muscle specific actin, and ultrastructurally by subplasmalemmal filament aggregates and micropinocytotic vesicles. Myofibroblastic differentiation has not been described before in dermatofibrosarcoma protuberans with fibrosarcomatous change and because of a lack of information concerning the immunohistochemical profile of this variant of DFSP its incidence is unknown. Focal myofibroblastic differentiation, as demonstrated in the fibrosarcomatous regions of these two tumors, might support the proposed dermal fibroblastic origin for dermatofibrosarcoma protuberans with fibrosarcomatous change.