What is a vaccine-associated fibrosarcoma?
Several studies have shown a strong association between the administration of feline vaccines (eg rabies and feline
leukemia virus) and subsequent development of
soft tissue sarcoma at the site of vaccination. The development of soft
tissue sarcoma at the sites of vaccine administration is estimated to occur in  1 out of 1,000-10,000 cats, and the time it
takes for the tumors to develop has been reported to range from 4 weeks to 10 years. A large study of cats in the United
States and Canada found that no single vaccine manufacturer or vaccine type is associated with the higher risk of cancer
development. The exact mechanism by which vaccination contributes to cancer development is not yet known, but it is
believed that the inflammatory reactions that occur after vaccination are responsible for the later tumor development.

Tumors that develop after vaccination are typically
fibrosarcomas, but can also include other sarcoma subtypes such as
rhabdomyosarcoma, malignant fibrous histiocytoma, undifferentiated sarcoma, extraskeletal osteosarcoma, and
chondrosarcoma. Vaccine-associated sarcomas are usually more aggressive in their behavior compared to other non-
vaccine sarcomas. One study of 100 cats diagnosed with vaccine-associated sarcoma reported that of these, 60% of cats
had high grade tumors (highly aggressive) compared to only 6% who had low grade tumors (mildly aggressive).

How is vaccine-associated fibrosarcoma diagnosed in cats?
The diagnostic tests for vaccine-associated sarcomas are similar to those described under the soft tissue sarcoma section.
imaging such as CT (computed tomography) or MRI (magnetic resonance imaging) are recommended for
vaccine-associated sarcomas to evaluate the extent of the disease in order to plan an appropriate treatment strategy.

Does cancer cause pain in cats?
Pain is common in pets with cancer, with some tumors causing more pain than others. In addition to pain caused by the
actual tumors, pets will also experience pain associated with cancer treatments such as surgery, radiation therapy or
chemotherapy. Untreated pain decreases the pet's quality of life, and prolongs recovery from the illness, treatment or
injury. It is, therefore, essential that veterinary teams that are taking care of pets with cancer should also play a vital role in
educating pet owners about recognizing and managing pain in their pets. The best way to manage cancer pain in pets is to
prevent it, a term referred to as preemptive pain management. This strategy anticipates pain ahead of time and
administers pain medication before the pet actually experiences pain, thus ensuring the pet's maximum comfort.

To learn more about which tumors are likely to cause a lot of pain, how to recognize pain in pets with cancer and what
cancer pain management options are available for your pet, please visit the
Cancer Pain Management section.

How important is nutrition for cats with cancer?
Cancer cachexia (a term referring to progressive severe weight loss) is frequently observed in pets with cancer. Pets with
cancer lose weight partly because of lack of appetite and partly because of cancer-induced altered metabolism. Some of
the causes for decreased appetite are related to the cancer itself (for example, tumors may physically interfere with food
chewing, swallowing, and digestion process) and some may be related to the side effects of cancer treatment (for example,
some chemotherapy drugs cause nausea and vomiting, and radiation therapy can cause mouth inflammation).

Proper nutrition while undergoing cancer treatment is essential to maintain your pet's strength, improve survival times,
quality of life and maximize response to therapy. Adequate nutritional support was shown to decrease the duration of
hospitalization, reduce post-surgery complications and enhance the healing process. Additionally, pets with cancer need to
be fed diets specifically designed to provide maximum benefit and nutritional support for the patient. To learn more, please
visit the
Cancer Nutrition section.

What are the treatment options for vaccine-associated fibrosarcoma in cats?
Vaccine-associated sarcomas are very invasive tumors and require an aggressive treatment including aggressive surgery
with or without
radiation therapy.

The surgical treatment removes not only the tumor itself, but also some of the surrounding healthy tissues to ensure that
no or very few cancer cells are left behind. When such healthy tissues are not removed, the tumor usually grows back  in
2.5 months. When the surrounding  tissues are removed, it takes on the average about 1 year for the tumor to grow back.
The first surgical attempt to remove vaccine-associated sarcoma has the best chance to control the tumor, therefore, it
should be performed by an experienced board certified surgeon. For tumors located on the limbs, amputation is usually
required to achieve sufficient control of the tumor.

Surgery and radiation therapy
Although surgery removes all of the visible tumor, if there are any cancer cells left behind, the tumor will grow back - a
process called local recurrence. Due to the high risk of local recurrence for vaccine-associated sarcomas,
therapy is recommended following surgery to attempt to kill any remaining cancer cells. The timing of surgery and radiation
therapy remains controversial among veterinarians. Administering radiation therapy prior to surgery has the advantages of
reducing the tumor size for subsequent surgery and decreasing the risk of spreading cancer cells throughout the surgical
site. Its main disadvantage, however, is that it increases the risk of surgical complications. Administering radiation after
surgery may provide better tumor control since individual cancer cells are easier to kill than an entire mass, however,
surgery increases the size of the area that will need to be irradiated. Radiation therapy should be scheduled within 10-14
days of surgery to maximize the benefit of this combinatorial approach.

The benefit of chemotherapy to treat vaccine-associated sarcoma is not well established. Some responses were observed
in cats treated with chemotherapy drugs doxorubicin, mitoxantrone, vincristine, and paclitaxel, but these responses were
usually short-lived. Administering chemotherapy after surgery/radiation therapy does not significantly affect the cats'
overall survival but may prolong the time the tumor will take to grow back.

How do I find a qualified veterinary oncologist?
To locate a qualified veterinary oncologist worldwide who can discuss with you appropriate cancer treatment plan for your
cat's cancer condition, please visit the "
Locate a veterinary oncologist" section.  

Are there any clinical trials investigating new treatments for vaccine-associated fibrosarcoma?
There are available clinical trials (which are partially funded by the institutions) investigating new treatments for vaccine-
associated sarcomas in cats in Colorado, Georgia, North Carolina and Wisconsin. To learn more about these clinical trials,
please visit the
Clinical Trials for Vaccine-associated Sarcoma section.

To learn more about veterinary clinical trials in general, please visit the
Pet Clinical Trials section.

What is the prognosis?
The prognosis for cats with vaccine-associated sarcoma treated by surgery alone is poor. Despite aggressive surgeries,
65% of cats develop new tumors one year after surgery and 91% after two years. The outcome is better for cats who are
treated with one initial aggressive surgery compared to those who are treated with multiple conservative surgeries. The
median average survival time following complete surgical tumor removal is approximately 16 months. Combining radiation
therapy with surgery improves the overall survival to 23 months but local recurrence still occurs in 28-45% of cats.
Metastasis (spread) to other organs such as the lungs has been reported for 12-26% of cats, with a median time to
metastasis of 265 days.

Additional online resources:
Vaccine-Associated Feline Sarcoma Task Force

  • Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
  • Morrison Wallace B. Cancer in Dogs and Cats: Medical and Surgical Management. Baltimore: Williams&Wilkins, 1998.
© 2007 Pet Cancer Center. ALL RIGHTS RESERVED.
Last updated 2/19/2017
Vaccine-Associated Fibrosarcoma in Cats
Comprehensive guide to cancer diagnosis and treatment in cats and dogs