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proliferative myositis

Tuesday 29 March 2005

PO

Definition: 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. It may be related to a reactive process. An occasional history of trauma can be noted.

The proliferative myositis belongs to the "pseudosarcomatous fasciitis and myositis" group, joining nodular fasciitis (NF), proliferative fasciitis (PF), and proliferative myositis (PM). They are pseudosarcomatous lesions that typically resolve spontaneously.

Images

- Proliferative myositis

- ganglion-like cells in proliferative myositis

- Proliferative Myositis w Osseous Metaplasia

Digital cases

- JRC:12028 : Proliferative myositis.
- JRC:1433 : Proliferative myositis.
- JRC:1914 : Proliferative myositis.

Clinical synopsis

- Painless mass that grows rapidly in 1 to 6 weeks
- Mean age 50 years, rare in children
- Muscles of trunk, shoulder, chest or thigh

Macroscopy

- 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

Microscopy

- fibrosed muscle fibers
- plump myofibroblast fibroblast-like cells within the fibrotic area
- scattered giant ganglion-like cells with deeply staining basophilic cytoplasm and prominent nucleoli
- plump fibroblasts and myofibroblasts surrounding individual muscle fibers creating a checkerboard pattern
- proliferative fibroblasts alternating with atrophic muscle
- large ganglion-like cells with abundant amphophilic to basophilic cytoplasm, vesicular nuclei and prominent nucleoli
- stroma collagenous or myxoid
- variable mitotic figures but no atypical ones
- ill defined margins
- +/- metaplastic bone

Differential diagnosis

- The proliferative myositis belongs to the "pseudosarcomatous fasciitis and myositis" group, joining nodular fasciitis (NF), proliferative fasciitis (PF), and proliferative myositis (PM). They are pseudosarcomatous lesions that typically resolve spontaneously.

- 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

- 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

- rhabdomyosarcomas

  • desmin+, myogenin+
  • ganglion-like cells of proliferative myositis lack cross striations and are more basophilic than rhabdomyoblasts

- sarcomas

  • large mass
  • marked atypia
  • atypical mitoses
  • necrosis

Cytogenetics

- t(6;14)(q23;q32) (9332481)
- trisomy 2 (7923069)

Management

- Conservative surgery is curative, may have spontaneous resolution (Head Neck 2007;29:416).
- Recurrence suggests diagnostic error.

Links

- Radiological case http://radiology.rsna.org/content/244/2/613.long

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