Enzinger and Weiss have defined  fibrosarcoma as  a  'malignant tumour of fibroblasts that shows no other evidence of cellular differentiation and is capable of recurrence and metastasis.'

Pathologists have become more cautious in the making the diagnosis of fibrosarcoma and over the years there has been marked decline in this diagnosis. 

With the assistance of the electron microscopy and immunohistochemistry pathologists are now confidently diagnosing conditions like spindle cell melanoma, spindle cell squamous cell carcinoma, malignant peripheral nerve sheath tumour or monophasic synovial sarcoma.

Some of these conditions were misdiagnosed as fibrosarcoma in the past.   

The two main main groups of fibrosarcoma are the Adult and Infantile types:

Age:   Presents usually in the fourth to sixth decades as a painful mass.

Site:  Fibrosarcomas can arise from connective tissues such as fascia, tendon, periosteum and scar. Thigh or trunk are commonly involved.

Gross features:  Well circumscribed mass. Adult fibrosarcomas are almost always deep seated.  There may be areas of necrosis or haemorrhage.

Microscopic features:   Monomorphic population of spindle cells arranged in fascicles and intersect at acute angles resulting in herringbone appearance.  Mitoses are common. Pleomorphism is minimal. The reticulin stain demonstrate abundant fibers wrapped around each cell.

Immunohistochemistry :  Tumour cells demonstrate positivity  for  vimentin and are negative for EMA (epithelial membrane antigen), cytokeratin, S100 protein, smooth muscle actin and desmin.

Image Link (ESCOP):Image1 Image2 

                             Image3   Image4   

Congenital- infantile fibrosarcoma morphologically resembles adult fibrosarcoma, but it is considered as a separate entity because of its markedly different clinical behaviour with more favourable prognosis.

This rare low grade malignant neoplasm is locally aggressive with an increased rate of local recurrence. Systemic metastasis is very rare.

Wide excision is the treatment of choice. The estimated 5-year survival rate for congenital-infantile fibrosarcoma is greater than 80%.  Recurrence and metastatic rate depends on the completeness and extent of local resection.

Age:  Occurs in the first two years of life. Up to 30% are present at birth. There is a male predominance.

Site: These are usually subcutaneous lesions and are commonly located in the distal extremities. Other less common sites include the trunk and the head & neck region. Rare tumours may occur in the mesentery, retroperitoneum and orbit.

Gross: Tumour size varies considerably from a few centimeters to replacement of the entire distal limb portion. The cut surface is grayish white with areas of necrosis and hemorrhage together with myxoid and cystic areas.

Microscopic features:  

Image Link (ESCOP): Image1 Image2      Image3  Image4  Image5

The tumour consists of relatively uniform spindle shape cells arranged in fascicles. These cells are more rounded and immature than the cells in the adult type. The spindle cells contain large hyperchromatic nuclei. Numerous mitotic figures are present. Hemangiopericytoma-like vascular pattern is noted in the backround.

Note:  Histologically, fibrosarcoma may be almost identical to fibromatosis except that it shows more cellularity, nuclear atypia, increased mitotic activity, and areas of necrosis.

Immunohistochemistry : Reveals positivity with vimentin . S100 protein is negative. In some cases there is  focal positivity with desmin, smooth muscle actin and cytokeratin.

Ultrastructurally fibroblastic and myofibroblastic differentiation have been demonstrated.

Cytogenetic analysis reveals t(12;15)(13p;q25) translocation (identical to mesoblastic nephroma).

Differential diagnosis -   In contrast to adult fibrosarcoma, infantile fibrosarcoma affects the more distal portions of the extremities rather than proximal extremities and the incidence of metastasis is low in the congenital- infantile form.

Fibromatosis: Cellularity varies, a herringbone pattern is usually absent, mitotic figures are rare, hemorrhage and necrosis are absent, and there are no characteristic cytogenetic abnormalities.   Fibrosarcoma is more cellular, mitotic figures are frequent, hemorrhage and necrosis are often present. Cytogenetic analysis is useful in distinction of cellular fibromatosis from congenital fibrosarcoma.

Other spindle cell tumours :  variants of synovial sarcoma, liposarcoma,  leiomyosarcoma, spindle cell rhabdomyosarcoma, infantile hemangiopericytoma, osteogenic sarcoma.

Molecular detection (using reverse transcription polymerase chain reaction assay or fluorescent in situ hybridization) of ETV6-NTRK3 gene fusion reliably differentiates this tumour from other childhood spindle-cell tumours.