Proliferative Myositis
Definition
- Intramuscular reactive process in which large ganglion-like cells, fibroblasts and myofibroblasts expand the spaces between muscle fibers
Diagnostic Criteria
- Intramuscular mass
- Usually 3 to 4 cm diameter
- Relatively normal muscle fibers separated by expanded myxoid connective tissue
- Produces distinctive checkerboard pattern
- No evidence of muscle damage
- No sarcolemmal proliferation
- No multinucleation
- No nuclear enlargement
- No necrosis
- Connective tissue space between fibers contains reactive cells resembling those seen in nodular fasciitis and proliferative fasciitis
- Spindled and stellate fibroblasts and myofibroblasts
- Vesicular nuclei
- No cytologic atypia
- Ganglion-like cells
- Characteristic feature of this process
- Large round regular nuclei
- Occasional binucleate and trinucleate cells may be seen
- Prominent nucleoli
- Dispersed chromatin
- Basophilic to amphophilic cytoplasm
- May be clustered and may mold each other
- Mitotic figures frequent
- No atypical mitotic figures
- Spindled and stellate fibroblasts and myofibroblasts
- Reactive bone or cartilage seen in 20% of cases
Richard L Kempson MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342
Original posting: March 15, 2008
Last update: June 15, 2008
