Myxoid dermatofibrosarcoma protuberans: morphological, ultrastructural and immunohistochemical features.

Two uncommon cases of dermatofibrosarcoma protuberans with prominent myxoid changes are presented. The tumors appeared as large multinodular cutaneous plaques that arose at the sites of excision of previous tumors some years earlier. In addition to limited fibrous storiform features, focally observed in deep and peripheral portions of the tumors, a diffuse myxoid pattern could be observed. The latter consisted of homogeneous areas of rare, stellate or spindle-shaped cells, haphazardly scattered in abundant myxoid matrix. Cells of myxoid neoplastic tissue showed mainly a positive immunoreaction for fibrohistocytic markers and the absence either of muscular, neural or human progenitor cell antigens. Mitotic figures were fewer and cell proliferation rates were lower in myxoid as compared to those of typical dermatofibrosarcoma protuberans used as a control. The ultrastructural examination of myxoid areas revealed a prevalent fibroblast-like cell population showing dilated cytoplasmic vesicles, sometimes containing glycosaminoglycans-like substances. The extent of myxoid changes together with the characteristic morphological, ultrastructural and immunohistochemical features confirm that myxoid dermatofibrosarcoma protuberans is a distinct variant of this fibrohistiocytic tumor to be considered in the differential diagnosis among myxoid tumors of the skin.