Fibrosarcomas are malignant tumors of fibroblasts. Fibroblasts are cells within the dermis of the skin that produce the collagen in connective tissue. These tumors are locally invasive, often recur locally, and have a tendency to spread to other parts of the body (metastasize). Metastasis often occurs weeks to months following surgical removal. Fibrosarcomas are hard to control through surgery even when ancillary therapy like chemo or radiation is employed.

Three forms of fibrosarcoma have been recognized in cats: virus-induced fibrosarcoma secondary to feline sarcoma virus, solitary fibrosarcomas in older cats, and post-vaccinal fibrosarcomas. Virus-induced fibrosarcomas are caused by Feline sarcoma virus (FeSV) and rarely occur. FeSV requires Feline Leukemia virus as a helper virus for the development of fibrosarcomas that are locally invasive and metastasize frequently. This type of fibrosarcoma is usually seen in cats less than five years of age which are feline leukemia positive.

Solitary fibrosarcomas typically occur in older cats. They form subcutaneous tumors that are usually seen on the trunk, limbs, digits, and pinnae (ears). These tumors are usually a low-grade malignancy that commonly recur locally and will metastasize infrequently.

Post vaccinal fibrosarcomas tend to occur 3 months to 3.5 years following vaccination in an estimated one of every 10,000 cats that are vaccinated. Post-vaccinal fibrosarcomas are aggressive and commonly recur multiple times within a period of weeks to months when removed. Metastasis occurred in approximately 25% of all cases, usually to the lungs or regional lymph nodes. Alterations in oncogene and growth factor expression are believed to be involved in the pathogenesis. The exact mechanism of fibrosarcoma development and the responsible vaccine component necessary for tumor development have not yet been fully determined. Vaccines to rabies and feline leukemia are more commonly implemented in tumor formation. Non-adjuvanted vaccines are touted as safer, but fibrosarcomas have occurred in cats following the use of a non-adjuvanted vaccination.

In dogs, fibrosarcomas are usually seen on the trunk and limbs. Fibrosarcomas in the dog are typically a low-grade malignancy that commonly recur locally, but will metastasize infrequently. A correlation has been noted between canine fibrosarcoma from presumed injection sites, but is seen on a much less frequent basis in the dog as compared to the cat.


References:
Maxie, M. Grant Ed. Pathology of Domestic Animals. Vol. 1: 5th Edition. 2007. Saunders/Elsevier. Pp. 764-765.

“Feline Adverse Vaccine Reactions”. The Compendium for Continuing Education for Veterinarians. Merial Limited. Vol. 29(10) October 2007. Pp.610 and 611.

“Vaccine Adverse Events”. Antech Diagnostics News. October 2007.