proliferative myositis

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

• https://twitter.com/yamahalion/status/746068675233353728
• https://twitter.com/JennyHoffmannMD/status/852249837319798784

 ganglion-like cells in proliferative myositis

• https://twitter.com/DrLaurenStuart/status/534754970177134594

 Proliferative Myositis w Osseous Metaplasia

• https://twitter.com/REP_Patologia/status/661286879220924416

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