Myofibrosarcoma (low-grade myofibroblastic sarcoma) with intracytoplasmic hyaline (fibroma-like) inclusion bodies.

Myofibrosarcoma is a controversial neoplasm composed of cells with differentiation toward myofibroblasts. The authors report an unusual case of myofibrosarcoma in which, in addition to the characteristic features reported, tumor cells contained intracytoplasmic hyaline (fibroma-like) inclusion bodies. A 66-year-old man complained of a painless enlarged mass in his right shoulder over the previous 6 months. The tumor recurred 2 years after operation. On histologic examination, the tumor displayed diffusely infiltrative growth with isolation of individual skeletal muscle fibers. The tumor cells were most often arranged in an intersecting (herringbone) fascicular pattern but a vaguely storiform pattern was also observed. Cellularity varied from one area to another. The cells were spindle-shaped, with ill-define pale eosinophilic cytoplasm, and a wavy or tapering nucleus with fine chromatin and small nucleoli. Intracytoplasmic hyaline inclusion bodies of variable size, often located adjacent to the nucleus, were visible in numerous spindle and stellate cells. The mitotic index was 3/10 HPF. The tumor was scored as grade 1. Immunohistochemistry revealed positive staining to muscle-specific actin, desmin and vimentin in most tumor cells. Intracytoplasmic hyaline inclusion bodies did not react specifically to any one antibody, but showed a marked ring-like immune reaction, particularly to muscle-specific actin. Electron microscopy showed tumor cells with indented nuclei and small nucleoli, abundant rough endoplasmic reticulum, micropinocytotic vesicles, and longitudinally arranged fine filaments with focal electron-dense patches and subplasmalemmal plaques. The most striking feature was the presence of large, globular or ball-like, non-membrane-bound, randomly scattered clusters of fine filament, usually adjacent to the nucleus.