Injection-Site Sarcoma in Cats

ISS tumors are firm growths underneath the skin that may start off small and continue to grow. Once the tumors are present, cancer can easily spread to other parts of the body. The quicker you bring your cat to a veterinarian for treatment, the better chance he has of surviving.

Vaccines and other injections used to administer medication are intended to protect your cat, but sometimes, they can lead to a very serious condition known as injection-site sarcoma (ISS). ISS is tumor growth that occurs at the site of an injection weeks, months, or years later. These tumors usually develop as a result of rabies or feline leukemia vaccines, however, any injection could cause them. Cat owners should frequently monitor injection sites including the legs and shoulder blades to check for any unusual growths.

The main symptom of an injection-site sarcoma is a tumor, or mass, around the location of the injection. But, it’s important to note the tumor can form long after the injection took place. Most tumors form within a few weeks, but they can form up to 10 years after the initial injection. The tumors are usually firm and located in injection sites between the shoulder blades or on the rear legs.

Although it is rare, it’s possible for a tumor to develop at an injection site. These tumors are most often linked to vaccinations, but they can occur after any type of injection. It’s unclear why these tumors form, however, many experts believe some cats are genetically predisposed to this condition.

If you notice any lumps on your cat, bring him to a veterinarian right away. Let your vet know when you first noticed the lump, and also give him a background of what vaccinations and injections the cat had in the past. Remember, tumors can take years to form, so it’s important to tell your vet about every injection your cat has had, not just the most recent ones.

Once you have discussed your cat’s symptoms, the vet will need to begin diagnostic tests. Needle aspirates are usually used to diagnose injection-site sarcomas. This involves inserting a small needle directly into the tumor to remove cells for testing. However, this test is not always accurate, so a biopsy may need to be performed to confirm a diagnosis. Vets usually follow the 3-2-1 rule when deciding whether a biopsy is needed. This rule states a biopsy is needed if the tumor has been present for at least 3 months, is wider than 2 cm in diameter, or if it increases in size after 1 month.

If the biopsy shows the tumor is cancerous, the vet may also perform a complete blood count and urinalysis test to determine if the cancer has spread. X-rays of the abdomen and chest may also be done.

Your cat will need to go through aggressive treatment to eliminate the cancer. First, the vet will most likely perform surgery to remove the tumor. Injection-site sarcomas usually affect tissue surrounding the tumor, so the vet will need to remove surrounding tissue that is not actually part of the tumor. If the tumor is on your cat’s leg, it’s common for the vet to recommend amputating the entire leg instead of performing surgery to remove the mass.

After the tumor has been removed, the vet may recommend your cat go through radiation, chemotherapy, or both to reduce the chance of another tumor developing. It’s possible your vet may also recommend radiation prior to the surgery to reduce the size of the tumor. 

Radiation is performed every day for around four weeks, and your cat will be anesthetized for each treatment. Radiation is only administered to the area that has been affected by the cancer, whereas chemotherapy is administered to the entire body. Chemotherapy is usually given in three-week intervals instead of daily like radiation treatment. Your cat will probably experience nausea, fatigue, and loss of appetite as a result of the chemotherapy, but radiation side effects are minimal.

All types of cancer are unpredictable, so it’s hard to predict whether your cat will recover from this condition or not. However, the earlier the cancer is detected and treated, the better chance your cat has of surviving. If the cancer has already spread to other parts of your cat’s body, the outlook is not as good. Because it’s possible for the cancer to come back after it has been treated, you will need to frequently check for new growths on your cat’s body. The vet may also recommend coming in on a regular basis so he can monitor your cat’s health.