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Fibrosarcoma of Bone

Fibrosarcoma of bone is a spindle cell malignant neoplasm of bone composed of relatively monomorphic fibroblastic tumor cells with variable collagen production and a fascicular (commonly herringbone) architecture. The diagnosis is one of exclusion.

Symptoms & Causes

Introduction

Fibrosarcoma of bone is a rare malignant tumor characterized by spindle-shaped fibroblastic cells forming a fascicular pattern, often requiring exclusion of other tumors for diagnosis.

Reference
WHO Classification of Tumours Editorial Board. Soft tissue and bone tumours [Internet]. Lyon (France): International Agency for Research on Cancer; 2020 [cited 2024 09 11]. (WHO classification of tumours series, 5th ed.; vol. 3). Available from: https://tumourclassification.iarc.who.int/chapters/33.

Related Terminology
None

Subtype(s)
None

Localization

The distal femur is the most common site (21–47%), with the proximal femur (16%), distal humerus (14%), and proximal tibia (11%) also involved.

Symptoms

The most common clinical signs and symptoms are local pain, swelling, limitation of motion, and pathological fracture.

Epidemiology

Historically, the term “fibrosarcoma of bone” has been applied to primary malignant spindle cell neoplasms of bone in which the tumor cells are typically organized in a fascicular or herringbone pattern. But a variety of primary bone tumors occupying other specific diagnostic categories may also show this histological pattern; therefore, there are no properties distinctive of or specific for fibrosarcoma of bone. This term is uncommonly used as a specific diagnostic category today, particularly due to the advent of ancillary techniques and evolving classification schemes. Therefore, the incidence is probably far less than the reported rates of as many as 5% of all primary malignant bone tumors. Tumors with a predominance of this fibrosarcomatous pattern, but probably representing a variety of tumor types, have been described with equal sex distribution and relatively uniform incidence over the second to sixth decades of life, with occasional occurrence in infants. Fibrosarcoma of bone represents a diagnosis of exclusion once specific lines of differentiation are excluded.

Etiology

Unknown

Diagnosis & Treatment

Staging

Staging is according to bone sarcoma protocols (see TNM staging of tumors of bone). See also the information on staging in section Bone tumors: Introduction.

Pathogenesis

There is little information in the literature regarding genetic changes in tumors with fibrosarcomatous differentiation. The average number of imbalances, detected by high-resolution array comparative genomic hybridization, is as high as 43 per tumor. The most common losses, observed by array comparative genomic hybridization, are 6q, 8p, 9p, 10, 13, and 20p; the most common gains are 1q, 4q, 5p, 8q, 12p, 15, 16q, 17q, 20q, 22, and Xp. CDKN2A (in 9p) is homozygously deleted in > 60% of 17 cases, and STARD13 (in 13q) is heterozygously or homozygously deleted in 45% of cases. Recently, a novel STRN-NTRK3 gene fusion has been reported in one case of primary fibrosarcoma of the radius. This finding, albeit rare, seems relevant considering the successful attempt to target NTRK with specific inhibitors.

Macroscopic Appearance

Grossly, fibrosarcomas are collagenous, resulting in a firm consistency with a trabeculated, white cut surface and circumscribed margins. High-grade lesions show a fleshy appearance.

Histopathology

Microscopically, fibrosarcoma is composed of a uniformly cellular population of spindle-shaped cells arranged in a fascicular pattern with a variable amount of collagen production. Given that these fibrosarcomatous features can be seen in a wide variety of bone tumors, the great majority of tumors previously considered to reside in this category are probably better placed in other categories, such as leiomyosarcoma, monophasic synovial sarcoma, and solitary fibrous tumor. Thus, fibrosarcoma is a diagnosis of exclusion. The presence of osteoid and cartilaginous differentiation excludes the diagnosis of fibrosarcoma.

Cytology

Not clinically relevant

Prognosis and Prediction

The 5-year and 10-year survival rates are as poor as 34% and 28%, respectively, depending on patient age and tumor grade, site, and stage, with metastases to the lungs and other bones commonly noted.

Clinical Features

Diagnostic Molecular Pathology

Molecular genetics plays a major role in excluding tumor-specific aberrations.

Essential and Desirable Diagnostic Criteria

Essential: bone tumor with compatible imaging; monomorphic cellular spindle cells; fascicular growth pattern; diagnosis of exclusion – lack of morphological, immunohistochemical, and genetic features suggesting an alternative diagnosis.

Desirable: herringbone fascicles.

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