Vaccine-Associated Sarcoma (Feline)

The term vaccine-associated sarcoma refers to a group of tumors that occur in some cats following the administration of vaccines or other injectable products. The tumors are thought to arise in these cats as a result of increased cellular multiplication associated with the inflammation that occurs in response to substances in the injected material.

Common name: Vaccine-associated sarcoma, injection-site sarcoma
Scientific name: These tumors are all labeled as vaccine-associated tumors. They are also named for the type of tissue involved in making up the tumor. i.e. vaccine-associated fibrosarcoma, vaccine-associated rhabdomyosarcoma, vaccine-associated osteosarcoma, etc.

Diagnosis

Signalment
Vaccine-associated sarcomas can occur in cats of almost any age, but are most common in 6 to 9 year old cats. These tumors can develop any time between 4 weeks to 10 years after vaccination.

Incidence/prevalence
Reports on the incidence of vaccine-associated sarcomas vary from 10/100,000 feline vaccinations to 130/100,000 feline vaccinations.

Geographic distribution
There is no known geographic predilection for this disease. It can be seen in any location in which cats are treated with injectable products.

Clinical signs(primary most to least frequent, scientific term, synonyms)
Presence of a palpable mass at an injection site. The mass is usually not painful, but may be ulcerated or infected if it is large.

Clinical signs (secondary,most to least frequent, scientific term, synonyms)
Most cats are presented due to the presence of a mass that has persisted for more than 3 months following vaccination. In rare cases in which the tumor has already metastasized at the time of presentation, the cat may have signs referable to the metastatic lesions i.e. difficulty breathing associated with tumor that has spread into the chest.

Cause (scientific, common term)
Injection-site sarcomas are thought to arise as a result of the inflammation caused by the injected material. The post-injection inflammation leads to uncontrolled multiplication of cells such as fibroblasts and myofibroblasts in the area around the injection site. This uncontrolled cell proliferation can lead to tumor formation in some cats.

The risk of developing a sarcoma increases as the number of vaccines given in one area increases.

Organ systems affected (most to least affected)
Subcutaneous tissue (tissue directly under the skin) in the area of a previous injection. These tumors are most commonly seen in region between the shoulder blades, over the back and along the sides of the chest and abdomen. They may be firmly attached to the underlying structures. Metastatic or secondary systems, lungs and chest, liver, spleen, kidney.

Diagnostic tests
Fine needle aspirate of the mass. Cytology of the aspirate may reveal signs of a tumor in addition to inflammation. Biopsy. A biopsy will usually demonstrate the presence of both tumor cells and inflammatory cells. The inflammatory cells (macrophage) sometimes contain bluish material which may be part of the vaccine.

Differential Diagnosis
Post-injection inflammatory reaction, abscess, other tumor.

Overview

The term vaccine-associated sarcoma refers to a group of tumors that develop in some cats following the administration of vaccines or other injectable products. These tumors can have many different names including fibrosarcoma, malignant fibrous histiocytomas, rhabdomyosarcoma, giant cell tumor, etc., but they are grouped together as vaccine-associated sarcomas because they all behave in the same manner and require the same treatment. They all likely occur as a result of the inflammation caused by injected material. It has been reported that for every 100,000 vaccines given to cats, between 10 and 130 tumors will develop. The tumors can occur in cats of any age, but are most common in cats between 6 and 9 years of age. The risk of developing these tumors increases as the number of vaccines given at a single site increases. Affected cats usually have non-painful masses located between their shoulder blades, along their back or sides or at any site of previous vaccination. The masses may be firmly attached to the body and may grow quickly.

Treatment

Home Care
Monitor sites of injections, especially vaccinations. Development of a swelling or mass (following injection) that meets any of the following criteria should merit evaluation by a veterinarian:

Mass greater than 2 cm in diameter.
Mass that persists for 3 or more months after injection.
Mass that increases in size a month or longer after injection.

Professional Care
The veterinary community is working very hard to minimize the risk of sarcoma formation for feline patients while still allowing for vaccination to protect cats from serious and sometimes fatal diseases. Some of the steps being taken include:

Working with vaccine manufacturers to develop vaccines that cause less inflammation and therefore hopefully result in less tumor formation.

Trying to vaccinate cats on their limbs where any tumor that develops can be treated more completely.

Educating clients about how to monitor vaccination sites.

Veterinarians can perform a needle aspirate or biopsy of a mass that develops following an injection. If a diagnosis of a vaccine-associated sarcoma is made, your veterinarian will develop a treatment plan for your pet that may include a combination of surgery, radiation therapy and/or chemotherapy.

Action
Most cats with vaccine-associated sarcomas will require surgery. The first surgery done on a feline patient must be an aggressive one since the extent of the first surgery is one of the most important factors affecting recurrence of the tumor. In some cases veterinarians may recommend CAT scans or other imaging procedures to help to determine the extent of the tumor and to plan surgery. Due to extensive invasion of these tumors into surrounding tissue, many patients will require radiation therapy in addition to surgery to delay recurrence of the tumor. Chemotherapy may also be recommended to delay the onset of metastatic disease and therefore prolong survival.

Outcome
With close monitoring and early diagnosis and treatment, many cats with vaccine-associated sarcomas can an excellent quality of life.

References/Additional Readings
Farrelly, J. Vaccine-site sarcoma. In: Cote, Etienne, ed. Clinical Veterinary Advisor Dogs and Cats. St. Louis: Mosby Elsevier, 2007; 1136-1138

Liptak, JM and Forrest, LJ. Soft Tissue Sarcomas. In: Withrow, SJ & Vail, DM, eds. Small Animal Clinical Oncology, 4th ed. St Louis: Saunders Elsevier, 2007; 425-454.

Moore, AnthonyS and Ogilvie, Gregory K. Soft Tissue Sarcomas. In: Ogilvie GK and Moore AS, eds. Feline Oncology A Comprehensive Guide to Compassionate Care. Trenton: Veterinary Learning Systems, 2001: 429-440

Author
Josephine McKnight, DVM
DACVIM (Oncology)

Editor
Sharon M. Gwaltney-Brant, DVM, PhD
DABVT, DABT