Fibrosarcoma In Cats

Fibrosarcoma in cats is generally considered by veterinarians to be one of our most frustrating cancerous

Fibrosarcomas can appear in cats of any age, but are more common as age increases.
Fibrosarcomas can appear in cats of any age, but are more common as age increases.


Fibrosarcoma (FSA) falls into a category of soft tissue sarcomas (STSs). STSs are cancerous growths that arise from the skin or subcutaneous (under the skin) tissues.

Triggers for soft tissue sarcomas are poorly understood (as is the case for so many cancers). Fibrosarcoma occurs spontaneously, however there is a known relationship between injection sites, especially vaccine injection sites, and FSA. Notice that I said “relationship” and not “cause.” Current knowledge does not allow us to say that injectables are actually causing fibrosarcomas in cats. Electron microscopy can confirm the presence of vaccine components, antibiotics and deworming agents, as well as other medications, inside some fibrosarcomas. The most commonly found vaccine ingredient present in feline FSAs is adjuvant. Adjuvants are ingredients added to medications to improve the body’s response to the medication. In the case of vaccines, adjuvants increase the immune system’s response to a vaccine. Research is ongoing to determine whether adjuvants may stimulate some cats to produce fibrosarcomas.

As stated above, fibrosarcoma occurs spontaneously in cats, with no other known cause other than a defect in cell division that triggers fibrous tissue cells to reproduce inappropriately. In addition, a defect in the immune system fails to find these abnormal cells and kill them.

Some lines, or families, of cats are known to produce fibrosarcomas more readily than others. Therefore, the obvious genetic relationship indicates that a certain component of the feline population will generate FSAs even if they never experience an injectable vaccine, antibiotic, or deworming agent.

Fibrosarcoma first appears on your kitty as a firm mass. It may achieve substantial size before it is first noticed because it begins in or under the skin. In medium- to-longhaired cats, the lesion may progress even further until it is large enough to be noticed when petting.

Treatment for FSA is usually surgical, at least initially. One troublesome characteristic of FSAs is that they form tentacles. Even when, to the naked eye of the surgeon, it appears that all of the tumor has been excised, abnormal cells may persist, allowing regrowth of the mass. To further complicate attempts to obtain clean margins, fibrosarcomas create a “pseudocapsule.” A truly encapsulated tumor creates a perimeter of tissue around itself that can allow a surgeon to literally “peel” a mass out if its home, intact. While a fibrosarcoma may appear to be encapsulated, those pesky microscopic tentacles are almost always present, unavoidably left behind, ready to continue growing.

FSAs can be treated initially with radiation when they are small. Larger tumors may be “debulked,” made smaller, by surgery before radiation.

Chemotherapy can be used as adjunct therapy in some cases.

The relationship between fibrosarcoma and vaccination has led leading cat practitioners to recommend cats be vaccinated at specific sites. The Vaccine-Associated Sarcoma Task Force (VAFSTF) asks veterinarians worldwide to administer Rabies vaccinations as low as possible on the right rear limb. Feline Leukemia Virus (FeLV) vaccines to be given as low as possible on the left rear limb. Vaccines for other diseases are to be administered near the right shoulder. Combination vaccines should be given in the rear limbs.

Specifically the interscapular space, the area between the shoulder blades, is to be avoided.

Why were these specific locations chosen by the Task Force? First, to provide a standard, so that if a fibrosarcoma develops we can know what vaccines may or may not have been given in the location of the tumor.

Second, because surgical intervention for FSA faces the obstacles stated above, the hope is that amputation of a limb might be curative by also removing the entire tumor and its tentacles. Fibroscarcomas on the trunk are virtually impossible to excise completely because tentacles invade deep layers, as well as spreading laterally.

Third, the interscapular region seems to be a “hotbed” location for FSA. Furthermore, if FSA develops there, little hope exists for complete surgical excision, as explained above. At one time this was a common area for veterinarians to administer vaccines.

Fortunately, fibrosarcoma is slow to metastasize to distant sites. Its persistence in regrowing locally more than makes up for that small ray of sunshine.

See you next week, Dr. Randolph.


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