Mast Cell Tumor in Cats

Mast cells are present in most tissues and can form an important part of your cat’s immune system. In the case of a mast cell tumor, the healthy cell begins to mutate and take on an abnormal shape and can grow and reproduce, invading other nearby tissue. Mast cell tumors can be benign (non-cancerous) or malignant (cancerous) and have varying degrees or ratings. Mast cell tumors have a tendency to affect older cats over 10 years of age and are especially prevalent in Siamese cat breeds. Mast cell tumors are also sometimes referred to as mastocytomas.

Like many tumors, mast cell tumors will begin with raised bumps or growths with an escalation of symptoms depending on the severity of the condition and whether the tumor is cancerous. Symptoms include:

• Raised growth or bump
• Loss of hair in area of bump
• Redness or irritation of bump
• Can be located on or below the skin
• Typically located on the trunk or body but can occur in other areas
• Ulceration may occur if your cat irritates the mass or in severe cases of cancerous tumor
• If a tumor metastasizes, or spreads, systemic symptoms such as lethargy, general sickness, or loss of appetite may occur.

The cause of mast cell tumors, as is the case with most types of tumors and cancers, is unknown. Mast cell tumors form and spread when an unnatural mutation occurs within the healthy mast cell. 

Diagnosis of mast cell tumor in your cat will begin with a thorough physical exam. Your vet will examine the suspected tumor and will search your cat for any additional tumors located on other parts of the body. You should provide a thorough medical and physical history of your cat to your veterinarian. It will be especially important to note the approximate date of first appearance of the tumor and the amount of growth since that time. If the tumor has changed in size rapidly, including reduction, this will be especially important for helping your vet determine how aggressive the tumor is.

Your vet will next conduct a biopsy of the tumor. This will often be done using a technique called fine needle aspiration. During this procedure, a needle is inserted into the tumor and is used to withdraw a small number of cells. Your vet will then view those cells under a microscope to determine if there are any cellular changes in appearance, also called differentiation, that could indicate the tumor is cancerous.

If your vet determines the tumor is malignant, he or she may order additional tests to determine whether or not the cancer has spread to any other organs. These tests may include a complete blood panel, fine needle aspiration of the lymph nodes closest to the tumor, and x-rays of the head and chest.

The preferred treatment for mast cell tumors in cats is surgical removal of the tumor. Prior to any surgery, your veterinarian will order a full blood panel in order to ensure there are no additional conditions that would make surgery extra risky for your cat. In preparation of surgery, your cat will be placed under anesthesia. Your veterinarian will attempt to remove as much of the tumor as possible. In the case of cancerous mass cell tumors, your vet will attempt to gain clean margins. This means they may have to remove extra tissue around the edges of the tumor to ensure that all of the cancerous cells have been removed.

In cases which cancer has spread, your veterinarian may opt to treat your cat with chemotherapy. During chemotherapy, strong drugs are administered to your cat. These drugs attack the cancer cells, causing them to shrink or disappear. Chemotherapy is usually administered in several treatments over a number of months and can have serious side effects. These side effects may be lessened by additional medications prescribed by your veterinarian.

In the case of benign mast cell tumors that have become inflamed, your vet may choose an alternative form of treatment involving anti-inflammatory drugs. Anti-inflammatory drugs will reduce the size and slow the spread of the tumor. This will be the treatment of choice for older cats who may be high-risk surgical candidates.

While they may appear frightening, mast cell tumors actually have a good prognosis for recovery. With surgical removal of the tumor, most cats will recover and lead normal healthy lives. As with any surgical procedure, you will need to carefully follow your vet’s post-operative instructions for wound care. You will also need to keep your cat quiet for several days following surgery.

While they can spread, cancerous mast cell tumors do not have a tendency to metastasize or move to other parts of the body. Your cat will need routine follow-up appointments with your veterinarian and careful monitoring on the owner’s part to ensure that tumors do not return or appear in other areas.