Fibrosarcoma of Bone

Fibrosarcoma of bone is a rare primary malignant spindle cell tumour that produces collagen but no osteoid or chondroid matrix. It makes up less than 5% of primary bone sarcomas, and it is a diagnosis of exclusion: it has no specific positive marker, so every other spindle cell tumour has to be ruled out first.

A shrinking diagnosis. Incidence has fallen dramatically over the past 30 years as immunohistochemistry and molecular testing have improved, which suggests that many historical cases were misclassified. Many tumours once called fibrosarcoma are now reclassified as undifferentiated pleomorphic sarcoma (UPS), synovial sarcoma, leiomyosarcoma or dedifferentiated chondrosarcoma, and the WHO trend is toward "undifferentiated/spindle cell sarcoma NOS". Whether true fibrosarcoma is a distinct biological entity or a diagnostic residuum remains debated.

Why it still matters. The diagnosis demands an extensive work-up to rule out other spindle cell tumours, a secondary fibrosarcoma signals malignant transformation of pre-existing bone pathology, and treatment differs from osteosarcoma, with a limited role for chemotherapy.

Who. Peak incidence is at 30-50 years, with a range of 10-80 years, and there is no significant sex predilection (1:1).

Where. The femur and tibia are the most common sites, and any bone can be affected:

• Femur 30%, tibia 25%, humerus 15%
• Axial sites (pelvis, mandible, spine) are uncommon
• Metaphyseal most often, sometimes diaphyseal
• Central (medullary) tumours are more common than surface (periosteal) ones

Primary and secondary. A primary fibrosarcoma arises de novo in normal bone. A secondary one arises in abnormal bone, and secondary tumours have a worse prognosis than primary ones. The substrates are:

• Paget's disease, the most common secondary cause, with a 1% sarcomatous degeneration rate
• Radiation, after a latency of 3-20 years, in the irradiated field
• Bone infarct, a rare substrate
• Chronic osteomyelitis (very rare), and fibrous dysplasia

The single most important distinction on this page. Infantile (congenital) fibrosarcoma shares a name and a herringbone architecture with adult fibrosarcoma, and differs from it in genetics, behaviour, treatment and, above all, prognosis. Confusing the two is a serious error in both directions: it either terrifies the parents of a child who will almost certainly do well, or produces dangerous complacency about an adult tumour.

Why the genetics change the operation. An ETV6-NTRK3-positive tumour responds to a TRK inhibitor. In an infant, then, the fusion is confirmed first, and if the tumour cannot be removed without amputating or sacrificing a critical structure, it is treated first and the surgeon operates into a smaller tumour. Fusion testing therefore has to happen before the operation is planned, not once the specimen comes back. The same logic now applies to any adult spindle-cell sarcoma found to carry an NTRK fusion, which is rare but actionable and is a reason to test when the histology is unclassifiable.

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Pathogenesis. Fibrosarcoma arises from primitive mesenchymal cells that differentiate along a fibroblastic lineage, producing collagen but no bone or cartilage matrix. The molecular pathogenesis is incompletely understood. Complex karyotypes are typical but non-specific, and there is no consistent chromosomal translocation, unlike synovial sarcoma or Ewing sarcoma, and that absence of a genetic signature contributes to the diagnostic difficulty.

What the microscope shows. Spindle cells, elongated fibroblasts with tapered nuclei, are arranged in fascicles that intersect at acute angles, the herringbone pattern. Collagen production varies, vascularity is variable without a specific pattern, and there is no osteoid or chondroid matrix. That absence is the defining feature: even focal osteoid excludes the diagnosis and suggests osteosarcoma, so extensive sampling is mandatory.

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A diagnosis of exclusion. Fibrosarcoma has no specific positive immunohistochemical marker. The diagnosis requires:

1. Spindle cell morphology with collagen production
2. Absence of osteoid or chondroid matrix on extensive sampling
3. Negative immunostains for other entities
4. Absence of specific genetic translocations

Immunohistochemistry and molecular testing are therefore used to exclude, and many historical cases have been reclassified as UPS or other specific entities this way.

Grade. Grade is the strongest prognostic factor, and it is read from cellularity, the organisation of the herringbone, nuclear atypia, mitoses, collagen and necrosis. Low-grade tumours carry the best outlook and high-grade the worst.

The FNCLCC grade. The Fédération Nationale des Centres de Lutte Contre le Cancer grade is the standard, reproducible sarcoma grade. It scores three parameters and adds them:

• Tumour differentiation, 1-3: 1 closely resembles normal adult mesenchymal tissue (well differentiated), through to 3 for undifferentiated, embryonal or uncertain-lineage sarcomas
• Mitotic count, 1-3: 1 = 0-9, 2 = 10-19, 3 = 20 or more mitoses per 10 high-power fields
• Necrosis, 0-2: 0 = none, 1 = under 50%, 2 = 50% or more
• Total: 2-3 = grade 1 (low), 4-5 = grade 2 (intermediate), 6-8 = grade 3 (high)

How the two fit together. The descriptive table above covers the features the FNCLCC formalises, and it is usually the mitotic count and necrosis that push a fibrosarcoma into the high-grade group. Several schemes are in use (FNCLCC, three-tier and older Broders-type systems), and the threshold for calling a tumour high grade and escalating treatment is not standardised. The grade feeds the Enneking stage (G1 versus G2) and the decision whether to consider adjuvant chemotherapy.

The limits of grading. The FNCLCC was derived largely for soft-tissue sarcoma and applied to bone by extension, interobserver variability in the differentiation score is real, and it performs less well for some translocation-associated sarcomas. Grade is therefore combined with the full histological and molecular assessment, not a substitute for it.

Radiographs. The plain radiograph is the initial assessment and contributes to surgical planning. It shows a destructive lytic lesion, geographic in low-grade tumours through moth-eaten to permeative in high-grade ones; the higher the grade, the more aggressive it looks. There is no matrix calcification or ossification, which would suggest osteosarcoma or chondrosarcoma. Periosteal reaction is variable and may include a Codman triangle in aggressive cases, and a soft-tissue mass is often visible once the cortex is breached.

Through a pathological fracture. The fracture is transverse or oblique through the lesion, with variable displacement. It may force a staged procedure, stabilisation and then resection, and whether it worsens the prognosis is unclear.

CT. Superior bone detail makes local CT essential for surgical planning. Acquire thin slices (1mm or less) with bone and soft-tissue windows and multiplanar reconstruction, and assess:

• The medullary destruction pattern
• Cortical breakthrough, its location and extent
• The size and margins of the soft-tissue mass
• The relationship to the neurovascular bundle
• Skip lesions, rare but important to detect

CT chest is mandatory for staging, because the lung is the most common metastatic site for bone sarcomas. Nodules over 5mm are suspicious, and the scan is the baseline for follow-up comparison.

MRI is the modality of choice for local staging, surgical planning and intramedullary extent. Its soft-tissue contrast shows involvement of muscle, nerve and vessels.

Whole-body staging and bloods. Whole-body imaging, PET-CT or a bone scan, looks for skip lesions and bone metastases, and PET-CT is an alternative route to whole-body staging. Blood tests (alkaline phosphatase and LDH) give a baseline and help exclude other pathology; alkaline phosphatase is usually normal unless there is Paget's disease.

Biopsy. Histological confirmation is mandatory before definitive treatment. The biopsy is performed by the treating surgeon, or in coordination with the surgical team, so that its trajectory can be excised at the definitive operation.

The pathology. Review by an expert musculoskeletal pathologist is mandatory. The specimen needs multiple sections to exclude osteoid, the immunohistochemistry panel, molecular studies if the diagnosis is uncertain, and a grade (low, intermediate or high).

Enneking (MSTS) surgical staging. The Enneking / Musculoskeletal Tumor Society staging of a malignant bone or soft-tissue sarcoma combines three axes. It is distinct from the AJCC TNM system, but it is the framework orthopaedic examiners expect for bone sarcoma.

• Grade (G): G1 low grade, G2 high grade (histological and biological aggressiveness)
• Site (T): T1 intracompartmental, confined within the bone or compartment of origin; T2 extracompartmental, through the cortex or compartment into surrounding tissue
• Metastasis (M): M0 none, M1 regional or distant metastasis

The axes combine into stages:

• Stage I (low grade, M0): IA = G1 T1 M0, IB = G1 T2 M0
• Stage II (high grade, M0): IIA = G2 T1 M0, IIB = G2 T2 M0
• Stage III: any G, any T, M1

Why the stage matters here. Most fibrosarcomas present as stage IIB, high grade with cortical breakthrough into a soft-tissue mass and no metastases. That is the dominant group in the Papagelopoulos series and the reason the standard operation is a wide en-bloc resection. A low-grade intracompartmental tumour (IA or IB) carries a much better outlook, while stage III disease shifts the goal toward systemic control.

Pain. The presentation is non-specific, like that of other primary bone malignancies. Progressive pain is the most common initial symptom, often present for weeks to months before diagnosis. Unlike the pain of benign lesions it is typically progressive and not related to activity. It is constant, moderate to severe, may be present at rest and may worsen at night, and responds poorly to NSAIDs; if a nerve is compressed, it may follow that nerve's distribution.

Other presentations. Systemic features (fever, weight loss) are usually absent, and neurological symptoms are rare unless a nerve is compressed.

• Pathological fracture in 10-20%
• Swelling or a palpable mass when there is soft-tissue extension
• Limited motion when the tumour is near a joint

Examination. Look for swelling, which is visible when the soft-tissue component is large, skin that is usually normal but may show venous prominence, an antalgic gait if the lower limb is affected, and possibly angular deformity after a pathological fracture. Feel for localised bony tenderness and a firm mass, fixed to bone if large or through the cortex; the temperature is normal, not warm as in infection. Assess distal pulses, sensation and motor function.

Movement and the regional examination. Range of motion may be limited by pain if the lesion is juxta-articular, strength is reduced if muscle is involved or by pain, and the limb is checked for a pathological fracture. Lymph nodes are usually not enlarged, because bone sarcomas rarely spread by lymphatics. Examine the chest for metastases, though these are rarely found on examination.

Suspecting a secondary tumour. The clues lie in the history of the bone.

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Osteoid decides it. The differential that matters most is dedifferentiated osteosarcoma. Osteosarcoma produces osteoid by definition, from malignant osteoblasts; it is more pleomorphic and its alkaline phosphatase is often elevated. It can contain dedifferentiated areas that mimic fibrosarcoma, which is why thorough sampling of multiple areas is essential.

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Survival. Grade is the strongest prognostic factor, but no cited study reports a modern grade-stratified figure for true fibrosarcoma of bone. The two largest series report 34% and 33.4% at five years overall, both drawn from cohorts spanning most of the twentieth century in which many cases would now be reclassified, and the only contemporary multimodal series (EURO-B.O.S.S.) contained just three fibrosarcomas among 113 patients.

Secondary tumours. Secondary fibrosarcoma does worse than primary, but the figures in the management table do not allow a like-for-like comparison: the Paget figure comes from a series that was 88% osteosarcoma, and the post-radiation figure covers every histology.

Surveillance. Recurrence and metastasis come early: 80% of local recurrences occur within the first 2 years and 90% within 5, and most metastases appear within the first 3 years, the lung being the most common site. Surveillance is more intensive for high-grade tumours and less so for low-grade ones.

• First 2 years: clinical examination and local imaging (X-ray or MRI) every 3 months, CT chest every 3 months
• Years 3-5: clinical examination and local imaging every 4-6 months, CT chest every 6 months
• After 5 years: annual follow-up with CT chest

Global Epidemiology

Fibrosarcoma of bone accounts for less than 5% of primary bone sarcomas worldwide, and reported incidence has fallen markedly over recent decades as immunohistochemistry and molecular testing reclassify many historical cases as undifferentiated pleomorphic sarcoma, leiomyosarcoma or dedifferentiated chondrosarcoma. It is best regarded as one of the rare primary malignant bone sarcomas (RPMBS), which collectively make up 5-10% of high-grade bone tumours (EURO-B.O.S.S.). Peak age is 30-50 years for primary disease, shifting older for secondary tumours arising in Paget disease or irradiated bone.

Society Guidance, Side by Side

Registries and Evidence Networks

• Reference-centre registries (e.g. Rizzoli/EMSOS in Europe, Mayo and Memorial Sloan-Kettering historical series) supply almost all outcome data, because population numbers are tiny at any single unit.
• Cooperative trial groups (EURO-B.O.S.S., COSS, Euro-EWING networks) pool rare bone sarcomas to generate prospective chemotherapy evidence — the basis for offering osteosarcoma-type regimens in fit high-grade patients.
• There is no implant survivorship registry endpoint specific to fibrosarcoma; reconstruction outcomes are extrapolated from limb-salvage endoprosthesis and allograft series.

High- versus Limited-Resource Practice