= PSEUDOMALIGNANT OSSEOUS TUMOR OF SOFT TISSUE = EXTRAOSSEOUS LOCALIZED NONNEOPLASTIC BONE AND CARTILAGE FORMATION = MYOSITIS OSSIFICANS CIRCUMSCRIPTA = HETEROTOPIC OSSIFICATION

= benign solitary self-limiting ossifying soft-tissue mass typically occurring within skeletal muscle as a mesenchymal response to soft tissue injury

◊Myositis is a misnomer for lack of muscle inflammation!

Cause: direct trauma (75%), paralysis, burn, tetanus, intramuscular hematoma, spontaneous

Age: 2^nd–3^rd decades; M >F

Path: lesion rimmed by compressed fibrous connective tissue + surrounded by atrophic skeletal muscle

Histo: hypercellular fibrous tissue with mature bone formation usually within 6–8 weeks after onset of symptoms

1. early stage: focal hemorrhage + degeneration + necrosis of damaged muscle → histiocytic invasion; central nonossified core of proliferating benign fibroblasts + myofibroblasts; mesenchymal cells enclosed in ground substance assume characteristics of osteoblasts → subsequent mineralization + peripheral bone formation
2. intermediate stage (3–8 weeks): “zoning phenomenon”:
   • central: cellular osteoid with atypical mitotic figures (impossible to differentiate from soft-tissue sarcoma)
   • middle: immature osteoid
   • outer: well-formed lamellar mature trabeculated bone
3. mature stage: shrinkage of mass → resolution in 30%

• pain, tenderness, soft-tissue mass

Location: large muscles of extremities (80%)

1. within muscle: head & neck (temporalis, masseter, buccinator, sternocleidomastoid); anterolateral aspect of thigh + arm; small muscles of hands; gluteal muscle; “rider's bone” (adductor longus); “fencer's bone” (brachialis); “dancer's bone” (soleus); breast, elbow, knee
2. periosteal at tendon insertion: Pellegrini-Stieda disease (in / near medial (tibial) collateral ligament of knee) as a result of Stieda fracture (= avulsion injury from medial femoral condyle at origin of tibial collateral ligament) [Augusto Pellegrini (1877–1958), surgeon in Florence, Italy] [Alfred Stieda (1869–1945), surgeon in Königsberg, Germany]

• gradual ossification from periphery toward center (!) of mass:
  • faint calcifications develop in 2–6 weeks after onset of symptoms
  • well-defined partially ossified soft-tissue mass apparent by 6–8 weeks, becoming smaller + mature by 5–6 months
  • radiolucent zone separating lesion from bone (DDx: periosteal sarcoma on stalk)
  • ± periosteal reaction

CT:

Early phase:

• well-defined geometric hypodense mass with peripheral calcification after 4–6 weeks + less distinct lucent center

Mature phase:

• diffuse dense ossification in mature lesion

MR:

• initially heterogeneous muscle edema
• progression to masslike region of high SI on T2WI (during first days to weeks after injury simulating malignancy)

Early phase:

• mass with poorly defined margins + surrounding edema
• inhomogeneously hyperintense to fat on T2WI
• isointense to muscle on T1WI
• contrast enhancement

Intermediate phase:

• isointense / slightly hyperintense core on T1WI, increasing in intensity on T2WI
• DIAGNOSTIC curvilinear hypointense rim surrounding the lesion (= peripheral mineralization / ossification)
• increased peritumoral SI on T2WI (= edema of diffuse myositis)
• focal signal abnormality within bone marrow (= marrow edema)

Mature phase: signal intensity characteristics of bone

• well-defined hypointense rim and trabeculae, dense fibrosis and central adipose tissue
• decreased SI inside and around lesion (dense ossification + fibrosis, hemosiderin from previous hemorrhage)

NUC:

• intense tracer accumulation on bone scan (directly related to deposition of calcium in damaged muscle)
• in phase of mature ossification activity becomes reduced + surgery may be performed with little risk of recurrence

Angio:

• diffuse tumor blush + fine neovascularity in early active phase
• avascular mass in mature healing phase

Prognosis: ? resorption in 1 year

DDx:

• In early stages difficult to differentiate histologically + radiologically from soft-tissue sarcomas!
  1. Osteosarcoma (densest calcification in center, least radiopaque bone at ill-defined periphery, NO surrounding edema)
  2. Synovial sarcoma
  3. Fibrosarcoma
  4. Chondrosarcoma
  5. Rhabdomyosarcoma
  6. Parosteal sarcoma (usually metaphyseal with thick densely mineralized attachment to bone)
  7. Posttraumatic periostitis (ossification of subperiosteal hematoma with broad-based attachment to bone)
  8. Acute osteomyelitis (substantial soft-tissue edema + early periosteal reaction)
  9. Tumoral calcinosis (periarticular calcific masses of lobular pattern with interspersed lucent soft-tissue septa)
  10. Osteochondroma (stalk contiguous with normal adjacent cortex + medullary space)