Fibrosarcoma (fibroblastic sarcoma) is amalignantmesenchymaltumour derived from fibrous connective tissue and characterized by the presence of immatureproliferatingfibroblasts or undifferentiatedanaplastic spindle cells in astoriform pattern. Fibrosarcomas mainly arise in people between the ages of 25 and 79.^[1] It originates in fibrous tissues of the bone and invades long or flat bones such as the femur, tibia, and mandible. It also involves the periosteum and overlying muscle.

Fibrosarcoma
Micrograph of a tumour with theherringbone pattern as may be seen in fibrosarcoma.H&E stain.
Specialty	Oncology

Individuals presenting with fibrosarcoma are usually adults thirty to fifty-five years old, often presenting with pain. Among adults, fibrosarcomas develop equally in men and women.^[2]

In infants, fibrosarcoma (often termedcongenital infantile fibrosarcoma) is usually congenital. Infants presenting with this fibrosarcoma usually do so in the first two years of their life. Cytogenetically, congenital infantile fibrosarcoma is characterized by the majority of cases having atranslocation between chromosomes 12 and 15 (notated as t(12;15)(p13;q25)) that results in formation of the fusion gene,ETV6-NTRK3, plus individual cases exhibiting trisomy for chromosomes 8, 11, 17, or 20.^[3] The histology, association with theETV6-NRTK3 fusion gene as well as certain chromosome trisomies, and the distribution of markers for cell type (i.e.cyclin D1 andBeta-catenin) within this tumor are similar to those found in the cellular form ofmesoblastic nephroma. Indeed, mesoblastic nephroma and congenital infantile sarcoma appear to be the same disease with the exception that mesoblastic lymphoma originates in the kidney whereas congenital infantile sarcoma originates in non-renal tissues.^[4][5][6]

The tumor may present different degrees ofdifferentiation: low grade (differentiated), intermediate malignancy and high malignancy (anaplastic). Depending on this differentiation, tumour cells may resemble mature fibroblasts (spindle-shaped), secretingcollagen, with rare mitoses. These cells are arranged in short fascicles which split and merge, giving the appearance of "fish bone" known as a herringbone pattern. Poorly differentiated tumors consist in more atypical cells, pleomorphic, giant cells, multinucleated, numerous atypical mitoses and reduced collagen production. Presence of immature blood vessels (sarcomatous vessels lacking endothelial cells) favors the bloodstream metastasizing. There are many tumors in the differential diagnosis, including spindle cellmelanoma, spindle cellsquamous cell carcinoma,synovial sarcoma,leiomyosarcoma, malignantperipheral nerve sheath tumor andbiphenotypic sinonasal sarcoma.^{[citation needed]}

Ancillary testing for fibrosarcoma includesIHC, wherevimentin is positive,cytokeratin andS100 are negative, andactin is variable.^{[citation needed]}

Fibrosarcoma occurs most frequently in the mouth in dogs.^[7] The tumor is locally invasive, and often recurs following surgery.^[8]Radiation therapy andchemotherapy are also used in treatment. Fibrosarcoma is also a rare bone tumor in dogs.^[9]

In cats, fibrosarcoma occurs on the skin. It is also the most commonvaccine-associated sarcoma.^[9] In 2014,Merial launched Oncept IL-2 in Europe for the management of such feline fibrosarcomas.^[10]

Bostock DE, et al. performed a study of cats that had fibrosarcomas excised and were followed for a minimum of three years, or until death. Two factors, tumor site and mitotic index, were found to be of prognostic significance, but tumor size, duration of growth, and histologic appearance were not. Following removal of fibrosarcomas from the flank in six cats, none died as a result of the tumor but 24 of 35 (70%) cats with fibrosarcoma in the skin of the head, back, or limbs were euthanized because of local recurrence, usually within nine months of surgery.^[11]

• Benign fibrous histiocytoma
• Dermatofibrosarcoma protuberans
• Fibrous connective tissue
• Fibroma
• Malignant fibrous histiocytoma
• Neurofibrosarcoma

• Fibrosarcoma of Bone: Review of a Rare Primary Malignancy of Bone
• Atlas of Pathology