Soft tissue

Fibroblastic / myofibroblastic

Proliferative myositis

Last author update: 1 June 2012
Last staff update: 31 July 2023

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PubMed Search: Proliferative myositis

Meggen Walsh, D.O., M.S., P.A.
Jerad M. Gardner, M.D.
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Cite this page: Walsh M. Proliferative myositis. website. Accessed November 29th, 2023.
Definition / general
  • Infiltrative poorly demarcated intramuscular mass resembling nodular fasciitis but with large basophilic cells resembling ganglion cells; histologically almost identical to proliferative fasciitis except located in muscle
  • Mean age 50 years, rare in children
  • Muscles of trunk, shoulder, chest or thigh
  • Painless mass that grows rapidly in 1 to 6 weeks
  • May be related to a reactive process, occasional history of trauma noted
  • Conservative surgery is curative, may have spontaneous resolution (Head Neck 2007;29:416), recurrence suggests diagnostic error
Gross description
  • Poorly circumscribed, scar-like induration of muscle, usually 3 - 4 cm, can occur under fascia, decreases the central portion of muscle in a wedge fashion
Microscopic (histologic) description
  • Cellular with plump fibroblasts and myofibroblasts surrounding individual muscle fibers creating a checkerboard pattern (proliferative fibroblasts alternating with atrophic muscle)
  • Also large ganglion-like cells with abundant amphophilic to basophilic cytoplasm, vesicular nuclei and prominent nucleoli
  • Stroma is collagenous or myxoid
  • Variable mitotic figures but no atypical ones
  • Ill defined margins
  • May have metaplastic bone
Microscopic (histologic) images

AFIP images

Characteristic checkerboard pattern

Metaplastic bone

Large ganglion cells

Abundant amphophilic cytoplasm

Spindle cell sarcoma

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Ganglion cells

Cytology description
  • Loose clusters of uniform fibroblast-like spindle cells and large, ganglion-like cells with eccentric nuclei, prominent nucleoli and abundant cytoplasm (Acta Cytol 1995;39:535)
  • Cytologic diagnosis not recommended due to limited sampling
Negative stains
Electron microscopy description
  • Fibroblasts and myofibroblasts, ganglion-like cells are fibroblasts or myofibroblasts with abundant dilated rough endoplasmic reticulum but no neuronal characteristics (Am J Surg Pathol 1991;15:654)
Differential diagnosis
  • Desmoid fibromatosis:
    • 3 cm or larger, completely replaces muscle
    • Spindle cells organized into broad sweeping fascicles, stroma is collagenous, skeletal muscle at periphery is often entrapped
    • No ganglion type cells
  • Ganglioneuroblastoma:
    • S100+
    • Similar histology but lacks the fibrillary background
  • Nodular fasciitis:
    • Completely obliterates muscle when extends deeper than fascia
    • No or few ganglion type cells
  • Proliferative fasciitis:
    • Almost identical except subcutaneous rather than intramuscular location
  • Rhabdomyosarcoma:
    • Desmin+, myogenin+
    • Ganglion-like cells of proliferative myositis lack cross striations and are more basophilic than rhabdomyoblasts
  • Sarcoma:
    • Large mass, marked atypia, atypical mitoses, necrosis
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