Leiomyosarcoma versus myofibrosarcoma: observations and terminology.

Two spindle cell sarcomas, which showed similarities in light microscopic histology and immunostaining and were diagnosed as leiomyosarcomas by these criteria, were compared ultrastructurally to show the value of electron microscopy in subtyping these neoplasms. Both were subcutaneous, case 1 occurring in the nasolabial fold and case 2 in the upper calf. Both consisted of fascicles of spindle cells, and both stained positively for vimentin and alpha-smooth muscle actin; only case 2 stained additionally for desmin. Case 1 showed strong and case 2 weak or negative staining for fibronectin. By electron microscopy, case 1 contained prominent rough endoplasmic reticulum, peripheral fine filaments with focal densities, and fibronexus junctions. By contrast, case 2 was characterized by an external lamina and well-developed bundles of fine filaments with focal densities. Case 2 was considered a typical leiomyosarcoma, and case 1 was interpreted as showing myofibroblastic differentiation. The nomenclature for these myofibroblastic tumors is discussed, and myofibrosarcoma and leiomyosarcoma, myofibroblastic variant are suggested as suitable terms.