= degenerative / reactive fibrous pseudotumor forming as a reaction to chronic mechanical friction

Frequency: in 24% of women + 11% of men >55 years (autopsy study); in 2% of CT population

Average age: 60 (range, 41–80) years; M÷F = 1÷4

Histo: enlarged irregular serrated elastic hypereosinophilic fibers, collagen, scattered fibroblasts, occasional lobules of adipose tissue; lack of capsule

• slow growing, often asymptomatic + clinically inapparent
• stiffness (25%), moderate pain (10%) if >5 cm in diameter
• clicking / snapping / clunking of scapula

Location: deep to serratus anterior + latissimus dorsi mm. at inferomedial border of scapula; R >L; bilateral in 10–66%

in <1% at: greater trochanter, olecranon, thoracic wall

PET: incidental finding

CT:

• heterogeneous poorly defined lesion of soft-tissue attenuation similar to muscle

MR:

• well-defined heterogeneous intermediate-SI lesion similar to muscle on T1WI + T2WI
• interspersed streaks of fat intensity in fascicular pattern
• heterogeneous enhancement

Rx: local excision for symptomatic lesions

DDx: extraabdominal desmoid, neurofibroma, malignant fibrous histiocytoma