Stanford School of Medicine
Surgical Pathology Criteria
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Proliferative Myositis


  • 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
  • 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


Supplemental studies


  • Ganglion-like cells
    • Factor XIIIa positive
    • Actin, CD68 negative to focal/weak
    • Desmin, keratin, S100, neuron specific enolase negative
  • Spindled/stellate cells
    • Actin positive
    • CD68 variable
    • S100, desmin negative

Differential diagnosis

Proliferative Myositis differs from Proliferative Fasciitis only by location


Proliferative Myositis Nodular Fasciitis
Ganglion-like cells present No ganglion-like cells
No damage to muscle fibers resulting in a checkerboard pattern with lesional cells between patches of intact muscle Muscle fibers obliterated or damaged
Nodular fasciitis may occasionally involve muscle


Fibromatosis, Abdominal Desmoid and Extra-abdominal Desmoid Proliferative Myositis
Muscle fibers damaged or obliterated Muscle fibers not damaged
Muscle fibers at edge of lesion Muscle fibers throughout
No ganglion-like cells Ganglion-like cells present
Predominantly collagenous Predominantly myxoid
Usually >3 cm Usually <4 cm
Both involve muscle


Rhabdomyosarcoma Proliferative Myositis
May have cross-striations No cross-striations
Cytoplasm typically eosinophilic Cytoplasm typically basophilic
Desmin, myogenin positive Desmin, myogenin negative
Cytologically malignant Lacks malingnant cytologic features
Atypical mitotic figures may be present No atypical mitotic figures
Usually >4 cm Usually <4 cm


Ganglioneuroma or Ganglioneuroblastoma Proliferative Myositis
Rarely involves skeletal muscle Involves skeletal muscle
Fibrillar background Myxoid background with reactive myofibroblasts
S100 may stain satellite cells S100 negative
Actin negative Ganglion-like cells may be actin positive


Malignant Neoplasm, NOS Proliferative Myositis
Muscle fibers damaged No damage to muscle fibers
Cytologically malignant Lacks malignant cytologic features
Atypical mitotic figures may be present No atypical mitotic figures
Usually >4 cm Usually <4 cm
Malignant neoplasms may occasionally separate muscle fibers in a checkerboard pattern


  • Usually middle aged or older
    • Rare cases reported in children
  • Usually short duration
  • Usually upper arm or trunk
  • Recurrence following excision is quite rare


  • Kempson RL, Fletcher CDM, Evans HL, Henrickson MR, Sibley RS. Tumors of the Soft Tissues, Atlas of Tumor Pathology, AFIP Third Series, Fascicle 30, 2001
  • Fletcher CDM, Unni KK, Mertens F. Pathology and Genetics of Tumours of Soft Tissue and Bone, World Health Organization Classification of Tumours 2002
  • Weiss SW, Goldblum JR. Enzinger and Weiss's Soft Tissue Tumors, 5th edition, 2008
  • Lundgren L, Kindblom LG, Willems J, Falkmer U, Angervall L.  Proliferative myositis and fasciitis. A light and electron microscopic, cytologic, DNA-cytometric and immunohistochemical study.  APMIS 1992 May;100(5):437-48
  • Ushigome S, Takakuwa T, Takagi M, Koizumi H, Morikubo M.  Proliferative myositis and fasciitis. Report of five cases with an ultrastructural and immunohistochemical study.  Acta Pathol Jpn 1986 Jul;36(7):963-71
  • Meis JM, Enzinger FM.  Proliferative fasciitis and myositis of childhood.  Am J Surg Pathol 1992 Apr;16(4):364-72.
  • el-Jabbour JN, Bennett MH, Burke MM, Lessells A, O'Halloran A.  Proliferative myositis. An immunohistochemical and ultrastructural study.  Am J Surg Pathol 1991 Jul;15(7):654-9
  • el-Jabbour JN, Wilson GD, Bennett MH, Burke MM, Davey AT, Eames K.  Flow cytometric study of nodular fasciitis, proliferative fasciitis, and proliferative myositis.  Hum Pathol 1991 Nov;22(11):1146-9
  • Choi SS, Myer CM 3rd.  Proliferative myositis of the mylohyoid muscle.  Am J Otolaryngol 1990 May-Jun;11(3):198-202
  • Enzinger FM, Dulcey F.  Proliferative myositis. Report of thirty-three cases.  Cancer 1967 Dec;20(12):2213-23
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