Desmoplastic fibroblastoma: a light and ultrastructural description of two cases.

The authors report 2 cases of collagenous fibroma in which ultrastructural analysis revealed the presence of fibronexus junctions, markers of myofibroblastic differentiation, never described in this rare lesion before. The tumors occurred in the trapezius muscle and in the right arms of a 41-year-old and a 25-year-old man. They were both intramuscular and showed sharp edges. Grossly, the excised masses were whitish and firm. Microscopically, they were both composed of stellate or spindle-shaped cells separated by a collagenous hypovascular and focally myxoid stroma. Mitotic figures and necrotic areas were not identified. Immunohistochemistry showed positivity for vimentin and focal positivity for smooth and human muscle actin, and flow cytometry showed the tumoral cells to be diploid.