Frequency: 4% of all primary bone neoplasm

Etiology:

1. PRIMARY FIBROSARCOMA (70%)
2. SECONDARY FIBROSARCOMA (30%)
   1. following radiotherapy of giant cell tumor / lymphoma / breast cancer
   2. underlying benign lesion: Paget disease (common); giant cell tumor, bone infarct, osteomyelitis, desmoplastic fibroma, enchondroma, fibrous dysplasia (rare)
   3. dedifferentiation of low-grade chondrosarcoma
      Histo: spectrum of well to poorly differentiated fibrous tissue proliferation; will not produce osteoid / chondroid / osseous matrix

Age: predominantly in 3^rd–5^th decade (range, 8–88 years); M÷F = 1÷1

Metastases to: lung, lymph nodes

• localized painful mass

Location: tubular bones in young, flat bones in older patients; femur (40%), tibia (16%) (about knee in 30–50%), jaw, pelvis (9%); rare in small bones of hand + feet or spinal column

Site: eccentric at diaphyseal-metaphyseal junction into metaphysis; intramedullary / periosteal

1. CENTRAL FIBROSARCOMA
   = intramedullary
   • well-defined lucent bone lesion
   • thin expanded cortex
   • aggressive osteolysis with geographic / ragged / permeative bone destruction + wide zone of transition
   • occasionally large osteolytic lesion with cortical destruction, periosteal reaction + soft-tissue invasion
   • sequestration of bone may be present (DDx: eosinophilic granuloma, bacterial granuloma)
   • sparse periosteal proliferation (uncommon)
   • intramedullary discontinuous spread
   • no calcification
   
   DDx: malignant fibrous histiocytoma, myeloma, telangiectatic osteosarcoma, lymphoma, desmoplastic fibroma, osteolytic metastasis
2. PERIOSTEAL FIBROSARCOMA
   = rare tumor arising from periosteal connective tissue
   Location: long bones of lower extremity, jaw
   • contour irregularity of cortical border
   • periosteal reaction with perpendicular bone formation may be present
   • rarely extension into medullary cavity

Cx: pathologic fracture (uncommon)

Prognosis: 20% 10-year survival

DDx:

1. Osteolytic osteosarcoma (2^nd–3^rd decade)
2. Chondrosarcoma (usually contains characteristic calcifications)
3. Aneurysmal bone cyst (eccentric blown-out appearance with rapid progression)
4. Malignant giant cell tumor (begins in metaphysis extending toward joint)