Soft tissue

Fibroblastic / myofibroblastic

Proliferative myositis

Last author update: 1 June 2012
Last staff update: 9 November 2021

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PubMed Search: Proliferative myositis [title]

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 June 4th, 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 is produced by reactive proliferation expanding spaces between muscle bundles and individual muscle fibers

With metaplastic bone, resembling myositis ossificans

Large ganglion cells with crowding and molding resembling carcinoma

Cells have abundant amphophilic cytoplasm and prominent nucleoli

Spindle cell sarcoma resembling proliferative fasciitis

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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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