Giant Cell Tumors in Cats

Giant cell tumor, more commonly known as malignant fibrous histiocytoma, is a subtype of fibrosarcoma and is usually malignant due to its rapid growth deep within the skin, its octopus-like shape, and the presence of spindle cells, which indicate cancer. Though rare in cats, malignant fibrous histiocytoma is the most common type of soft tissue tumors in cats, making up 60% of all sarcoma diagnoses. In rare cases, it can spread to muscles and to the lymphatic system. There is a high recurrence rate even after surgery.

Giant cell tumors are frequently found in the skin and connective tissues of middle to older aged cats such as at vaccine injection sites between the shoulder blades or in the soft tissue of the hind legs. They can also be found in the neck, abdomen, and forelimbs. There are no indications that giant cell tumors affect one sex more than the other.

All organs in the body have fibrous connective muscle tissue, nerves, and special white blood cells called macrophages or histiocytes. These blood cells work with the immune system to help eliminate harmful waste from the body and defend against disease. When there is an abnormal growth of these cells, benign tumors called histiocytomas may develop. 

There are also connective tissue cells called fibroblasts that help the body to heal after injury. If a histiocytoma is found to have fibroblasts fused to it, as well as additional multinucleated cells within the presence of a large amount of collagen, it is called a giant cell tumor and is usually malignant. Its development is often due to the immune system attacking an infectious agent cell.

Tumors under the skin or fat layers may appear in different sizes and shapes, but they are almost always firm and fleshy to the touch. Clusters of lumps can appear in one location. Other symptoms include:

• Loss of appetite
• Rapid weight loss
• Fatigue and lethargy
• Hard patches between the shoulder blades or in the soft tissues of the hind legs
• Bleeding, ulcerated skin overlaying lumps under the skin
• Labored breathing
• Abdominal swelling (depending on the location of the tumor)
• Difficulty urinating or having bowel movements
• Low blood sugar

The reason any cancer develops is unknown. Many doctors believe that it may be the result of a series of unfortunate issues that the pet is experiencing, some environmental and some genetic. Therefore, the true cause of malignant fibrous histiocytoma is also unknown. However, there are two strong suspicions of causes:

Recombinant Feline Leukemia Virus

Malignant fibrous histiocytoma may be instigated by feline sarcoma retrovirus (a variety of feline leukemia virus). This theory applies mainly to middle-aged cats but not older, cats since they have developed an immunity to the virus over time. Therefore, if an older cat is found with a tumor, it is highly probable that it is not related to the virus.

Manufactured Vaccines

Since tumors sometimes develop at injection sites for certain rabies and feline leukemia vaccines, manufacturers are testing to see if their vaccines are a contributing factor. Studies address whether the vaccines can be administered less frequently and if there is a safer location on the body in which to give the vaccine than between the shoulder blades, on the lower neck, or in the leg.

After an initial physical exam, your veterinarian will collect whatever evidence can be found to determine the cause of the lump. Typically, x-rays and ultrasounds are taken to show the location of the lump as well as its size and shape. Additional testing such as a blood test and a urinalysis may be conducted to rule out any underlying conditions. Note that these tests will not give an indication if the lump is cancerous or not.

Because a tumor can have varying characteristics, both between different tumors and even within the same tumor, it is very important to test for its exact type in order to identify what kind of cancer your cat has. For that reason, your veterinarian will conduct either a needle aspiration or a biopsy to make an accurate diagnosis. 

Needle aspirations are common, but they are rarely accurate. The only way to make a true determination of the type of cancer is to take a biopsy of the lump and send it to a specialized laboratory for testing. Because of the octopus-like structure of the tumor, in which its “legs” burrow deep into the tissue, exploratory surgery is necessary to obtain a viable sample of the lump. The laboratory will determine if spindle (cancerous) cells are present in the sample. If the entire lump was taken out, the laboratory will determine if all of the cancer cells were removed from the area and that the margins, or the area around the tumor, are clean.

After the laboratory has finished testing, a histopathology report is generated to help the veterinarian determine a course of treatment. The report will show the type of tumor the lump is, what its prognosis is, and what the probability of recurrence will be. 

If the tumor is confirmed to be cancer, rapid and aggressive surgical removal of the tumor and the surrounding deep tissue by a board-certified surgeon is necessary to have the best chance of success. If the tumor is located on a limb that is determined to be untreatable, the surgeon may amputate the limb to completely eradicate the cancer.

It is sometimes difficult to determine if the entire tumor has been removed, which means the chance of recurrence is very high. Recurrence usually indicates that the cancer has invaded deeper into the tissues and may be spreading. Additional surgery after recurrence does not necessarily increase your cat’s chance of survival. As a result, your veterinarian may choose to use radiation treatment either before or after surgery to lessen the probability of recurrence. Chemotherapy may also be administered if the tumor is very large, has metastasized, or cannot be removed due to the level of its invasiveness. 

Radiation treatment appears to only have an impact if it is given in high doses, which is why chemotherapy is becoming more popular. Although chemotherapy is not often a complete cure, it may add years to your cat’s life and provide a higher quality of life.

After surgery, it is very important for you to keep the incision site clean. A collar may be placed on your cat to prevent it from interfering with its healing. 

If you notice any swelling or bleeding around the incision site, or if there are sutures missing, call your veterinarian right away. 

Medications to address symptoms related to any underlying conditions will be administered to make your cat more comfortable. 

Side effects due to radiation treatment and/or chemotherapy are possible. Conduct ongoing conversations with your veterinarian for the best possible treatment of these effects.

The prognosis of a malignant fibrous histiocytoma tumor will depend on its location and how fast it is growing. Although most do not metastasize, they are usually locally invasive and difficult to fully remove during surgery, which means the probability of recurrence is high. More than 70 percent recur within one year of surgery. If the tumor is determined to be vaccine-associated, the rate of recurrence is 90 percent. The histopathology report generated by the laboratory will give specific expectations of recurrence or metastasis for your cat.

It is important to remember that the overall risk of your cat having malignant fibrous histiocytoma is extremely low. Current estimates are that the risk is from 1:1,000 to 1:10,000 that your cat will have a tumor of this kind. Protecting your cat against disease through the use of vaccines is still highly recommended and far outweighs the risk of cancer.