Vaccine Associated Sarcomas in Cats (VAS)

By Dr. Gail Mason, DVM, MA, DACVIM and Kathi Smith, RVT, Internal Medicine & Oncology Technician

Vaccinating domestic cats against serious infectious disease is an important step in good quality health care. The diseases that are vaccinated for can cause serious illness and/or death. Vaccination against rabies is an important pet health, public health, and legal issue as well. However, due to statistical association between some vaccines and subsequent sarcoma development which was noted over the last decade, vaccine types and protocols are under heavy scrutiny.

Injection site sarcomas, as the name implies, can develop in areas of the body where vaccination has taken place. This typically is between the animal’s shoulder blades (interscapular) or on a hind limb. These tumors are aggressive in nature and are associated with significant tissue inflammation and decay. They tend to infiltrate the normal tissues with malignant tendrils making surgical excision difficult. Although they are considered to be in the class of “soft tissue sarcomas,” they have a higher metastatic rate than other tumors of this class.

Current recommendations suggest that masses noted at vaccine sites which are present 3 or more months post-vaccinations, are greater than 2cm or are rapidly growing should be dealt with in a timely fashion. An incisional biopsy can confirm a VAS. Metastasis can occur to local lymph nodes and to the lungs.

Staging of the patient to evaluate the extent of the disease may include:
• Chest radiographs (x-rays)
• Abdominal radiographs
• Routine blood screening
• Ultrasonography or a CT scan

Patients have longer survival times when an aggressive, initial resection is performed and is followed by radiation therapy. If the tumor is located on a limb, it may be necessary to perform a limb amputation. As unpleasant as this idea is, it is well-documented that patient recovery is rapid and quality of life can be excellent. Chemotherapy has also been used in the treatment of VAS but its exact efficiency remains to be determined. Most published veterinary medical reports have described the use of either doxorubicin and/or carboplatin.

Unfortunately, cats with VAS may have a guarded to poor long-term prognosis. Remission times are variable, but most often reported as being between 8 months to 1.6 years. Initial aggressive and complete resection appear to positively influence outcomes and longer, excellent quality remissions have been documented.