Solitary infantile myofibromatosis of bone. An immunohistochemical and ultrastructural study.

A rare case of solitary infantile myofibromatosis of bone in an 11-month-old boy is reported. Radiographically the lesion of parietal bone was round, well-circumscribed, and osteolytic with a sclerotic rim. Histologically the tumor was made up of nodules that were hyalinized or cellular and containing plump, spindle-shaped cells that were intermediate in appearance between fibroblasts and smooth-muscle cells, arranged in short bundles or whorls. Another typical feature was the presence of distended, cleft-shaped vascular spaces around the nodules. The microscopic features of this tumor were consistent with those of infantile myofibromatosis of other sites, such as the skin and deep soft tissue. The tumor cells showed immunoreactivities for vimentin and alpha-smooth muscle actin. Microfilaments with dense bodies were observed in the fibroblast-like tumor cells. In addition, many tumor cells stained for collagen type IV and were covered by incomplete external laminae, indicating infantile myofibromatosis has more advanced smooth-muscle differentiation than conventional fibromatosis.