If you notice a lump on your cat, a fibrosarcoma is one of many possible causes. A fibrosarcoma is a type of soft-tissue sarcomas, a group of tumors that most commonly comes from soft tissues like fat, nerves, blood vessels, muscles, or tendons. Fibrosarcomas are rare cancerous tumors that typically develop on or under a cat's skin. The lump will likely be located on the hip, back, or shoulder blades and will be hard to the touch. Sometimes these may develop around injection sites. In the beginning, the lump won't be painful, but if untreated and metastasized, your cat will become very sick.

Learn more about what causes these soft-tissue tumors in cats and common treatment plans.

What Are Fibrosarcomas and Injection Site Sarcomas?

A fibrosarcoma is a type of cancerous, soft-tissue tumor that can occur in cats. Soft tissue sarcomas usually form within connective tissue, muscle, or fat under a cat's skin but can also develop in the mouth, nasal passages, and rarely on the spleen. The term "sarcoma" accounts for several different types of tumors. These tumors can grow quickly but, more often than not, are slow to develop. Most fibrosarcomas are very invasive in local tissues, but the risk of spread to other organs is relatively low. Even high-grade, advanced tumors metastasize in only 25% of cats.

Injection-site sarcomas often form at a place on your cat's body where a vaccine has been given. Though the reason for this isn't entirely clear, studies also show that in some cats, the process of receiving any substance into the skin is enough to induce inflammation related to the development of fibrosarcoma.

Symptoms of Sarcomas in Cats

The clearest indication of fibrosarcoma in a cat is the presence of a lump below the skin. There can be other types of lumps besides sarcomas. Inflammation and swelling can occur at the site of vaccines as the body mounts a local immune response that may not be a type of tumor but should be monitored carefully, and checked by a veterinarian if still present four weeks after vaccination. Lumps can vary in presentation and are sometimes accompanied by other symptoms.

Lump Under the Skin

A lump under your cat's skin is the best indicator of possible fibrosarcoma. A fibrosarcoma lump is firm and usually feels fixed to the tissue below the skin. The mass is likely not painful to your cat but can become infected and ulcerated if untreated. These lumps commonly occur at vaccine sites, so pay attention to growths at areas on the cat's body where vaccines have been administered, such as the shoulder blades, hips, limbs, and tail.

Oral Bleeding

Sometimes, fibrosarcomas can develop in a cat's oral tissue, and the masses can break open and bleed. Cats with oral fibrosarcoma may experience tooth loss, excessive drooling, and inability to eat.

Nasal Congestion and Discharge

Fibrosarcoma can form in a cat’s nasal passages, which can cause congestion, sneezing, and difficulty breathing. This can eventually cause a nasal discharge that may or may not have a blood-tinged appearance.

Systemic Symptoms

If fibrosarcoma has metastasized or is not treated, your cat will likely exhibit systemic signs beyond the lump, such as lethargy, decreased appetite, difficulty breathing, or pain. Without the presence of a lump, these symptoms are not necessarily related to cancer. Splenic fibrosarcoma typically goes undetected until the mass ruptures and causes internal bleeding. This may cause a sudden onset of pale gums, rapid heart rate, weakness, and collapse. The abdomen will begin to appear bloated as the internal bleeding continues.

Causes of Fibrosarcomas and Injection Site Sarcomas

There are several suspected causes of sarcomas, but there is usually no single explanation, like most types of cancer.

• Vaccinations: The administration of a vaccine can be correlated to the development of an injection-site fibrosarcoma in cats.  These tumors may not appear for years but are often at the vaccine site. It is thought that the local inflammation caused by the vaccine or the adjuvant in the vaccine can contribute to the formation of fibrosarcomas, but it is still not fully understood. Despite this correlation, the risks of not vaccinating your cat are greater than those of developing a vaccine-site-related fibrosarcoma. Vaccine-induced sarcomas are rare, and seem to be associated only with certain vaccines, so discuss the risks with your veterinarian. In most cases, the benefit of vaccines outweighs the relatively rare risk of sarcoma.
• Foreign bodies: Materials lodged under a cat's skin for a long time, such as a microchip or non-absorbable suture left behind in a wound, could cause inflammation that eventually results in fibrosarcoma.
• Feline sarcoma virus: Feline sarcoma virus (FeSV) is a rare hybrid virus that can cause an extremely fast-growing type of fibrosarcoma. Typically, this virus occurs in young cats that have also contracted Feline Leukemia Virus (FeLV).

How Vets Diagnose Sarcomas in Cats

To properly diagnose your cat's growth, your vet will perform a physical exam and administer tests like chest x-rays, bloodwork, CT scans, and biopsies. Typically, the diagnosis begins with a fine needle aspirate, when a needle is inserted into the mass to obtain a cell sample for evaluation. Depending on the stage of the cancer, your vet will determine the best diagnostic plan for your cat.

How to Treat Injection Site Sarcomas in Cats

Treatment for fibrosarcoma and other soft tissue cancers usually necessitates surgery, which may be followed by radiation therapy or chemotherapy. Mass removal surgery isn't always effective in removing cancer entirely because of the extent of the invasiveness of the tumor. Often, amputation is the best method for guaranteeing the complete elimination of cancer. Even when surgery is effective, some vets recommend proceeding with chemotherapy for additional protection, though the most effective type of chemotherapy hasn't been fully established. If the fibrosarcoma has spread, it will be much more challenging to treat.

Prognosis for Cats With Injection Site Sarcomas

The prognosis for cats with injection site sarcomas varies based on the stage and spread of the mass. Typically, fibrosarcomas are difficult to treat, and a cure is unlikely. Like all cancer treatments, the earlier a tumor is detected and treated, the longer your cat is likely to survive. Without complete removal of the fibrosarcoma with surgery and radiation therapy, it is very likely to recur within a year. However, if the tumor is removed completely, there is a 50% chance that it won't come back, even after several years.

How to Prevent Sarcomas

Because of the correlation between injections and sarcomas, it's beneficial to discuss which vaccines are necessary for your cat with your veterinarian. Following your veterinarian's recommendations for vaccines will help limit the potential of injection inflammation that could lead to fibrosarcoma. However, you should not avoid vaccinating your cat in general.

While not preventative of injection-site fibrosarcoma, the AAFP Feline Vaccination Advisory Panel recommends administering vaccines on a cat's lower limbs and tail limbs rather than the back or hips. This allows for the option of amputation if a fibrosarcoma does develop.

Some owners choose not to put microchips in their cats to avoid inflammation that may lead to an injection-site fibrosarcoma. There are alternatives to microchips like GPS tracking collars, but a microchip is still the most reliable way for others to identify your cat.