Injection-Site Sarcoma (Vaccine-Site Sarcoma) in Cats

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Feline Injection-Site Sarcoma (Vaccine-Site Sarcoma)

An injection-site sarcoma, also known as vaccine site sarcoma, vaccine-associated fibrosarcoma, and vaccine associated sarcoma, is a tumor thought to be induced by an injection – most often a vaccination. Post-vaccinal sarcomas are very rare but may occur in cats as a consequence of an overzealous inflammatory or immune system reaction to the vaccine.

A sarcoma is a malignant tumor composed of cells derived from connective tissue. These tumors often develop quickly and can spread (metastasize) to distant locations in the body. These tumors often are not responsive to treatment and result in serious illness and ultimately death of the animal. Recurrence of such tumors is common after surgical removal.

Injection-site sarcomas were first recognized in the late 1980’s when some changes occurred in the vaccine manufacturing process. At that time, manufacturers changed from production of modified live virus vaccines to killed virus products as directed by the United States Department of Agriculture (USDA). This change in manufacturing process resulted in the inclusion of aluminum into vaccines. It is this aluminum component of the vaccines that is suspected to be associated with development of post-vaccinal sarcomas. The feline leukemia virus and rabies vaccines are most frequently suspected in pets that develop post-vaccinal sarcomas.

The actual incidence of injection-site sarcomas is not known with certainty. Some investigators estimate that post-vaccinal sarcomas occur in as many as 1 of every 1,000 to as few as 1 in every 10,000 cats vaccinated. Injection-site sarcomas are recognized primarily in cats. The average age for onset of vaccine-site sarcomas is 7 to 9 years. There is no known breed predisposition. It is believed that tumors develop week to years after injection.

Despite the localized appearance of these tumors, microscopic branches of the tumor extend like fingers into the surrounding healthy tissue. During surgery to remove the tumor, these microscopic branches can remain and contribute to re-growth of the tumor. According to one study, as many as 62 percent of post-vaccinal sarcomas recur within 6 months after surgical removal.

There are several types of injection-site sarcomas:

  • Fibrosarcoma – a malignant tumor arising from cells called fibroblasts in connective tissue. This is the most common type.
  • Histiocytoma – a malignant tumor arising from tissue cells called histiocytes
  • Osteosarcoma – a malignant tumor arising from bone
  • Chondrosarcoma – a malignant tumor arising from cartilage
  • Rhabdomyosarcoma – a malignant tumor arising from muscle
  • Myxosarcoma – a malignant tumor arising from loose connective tissue
  • Liposarcoma – a malignant tumor arising from fat
  • Neurofibrosarcoma – a malignant tumor arising from cells called fibroblasts in connective tissue and nerve tissue.
  • Hemangiopericytoma – a malignant tumor that arises from the pericytes, cells that are located around vessels.
  • Schwanoma –are tumors of the nerve sheath
  • Leiomyosarcoma – a tumor arising from smooth muscle.
  • Nerve sheath tumor – a tumor arising from the deep soft tissue, usually in close proximity of a nerve trunk
  • Myofibroblastic sarcoma – a malignant soft tissue tumor in which myofibroblasts are quantitatively the predominant cell type.
  • What To Watch For

    You should watch for a firm, painless swelling in a subcutaneous (under the skin) location in the region of the body in which the cat was vaccinated or received an injection. It is wise to run your hand over your cat’s shoulders, back and rear legs periodically to monitor for development of abnormal lumps or tumors. The mass maybe hairless or ulcerated in some cats.

    Diagnosis of Injection-Site Sarcoma (Vaccine-Site Sarcoma)

    A swelling, nodule or mass under your cat’s skin in a region of the body that was previously used for injection or vaccination should be taken very seriously. If this occurs, you should schedule an appointment with your veterinarian to have your cat examined and the mass evaluated. This approach is recommended especially if the mass has persisted for 3 or more months, is larger than 2 cm (just under one inch) in diameter, or if you notice that the mass has been increasing in size during the one month after injection or vaccination. Have your veterinarian examine your cat as soon as possible in this situation.

    Diagnostic tests are needed to recognize injection-site sarcomas and exclude other diseases.

  • A complete medical history and thorough physical examination. An injection-site sarcoma is suspected based on a history of vaccination (or other injection) in the location on the cat’s body in which the tumor has been identified.
  • Fine needle aspirate. A fine needle aspirate inovles placing a needle into the mass and sucking back tumor cells. The cells are injected onto a microscope slice and examined. This can help identify the tumor type to help determine the stage of the tumor and the treatment recommendations. Fine needle aspiration not considered reliable for the diagnosis because often this type of tumor does not readily shed cells during routine needle aspiration. A biopsy is often preferred as a first step for diagnosis of the mass.
  • Biopsy. A swelling that develops at the site of a previous vaccination or other injection should be considered malignant until proven otherwise. Such a swelling should be subjected to surgical biopsy and microscopic examination if the mass has been present for 3 or more months, if the mass is larger than 2 cm (just under one inch) in diameter or if the mass is increasing in size one month after injection.

    Biopsy is a procedure in which a small portion of the mass is removed, preserved in a fixative solution, and sent to a laboratory for microscopic examination by a veterinary pathologist. Biopsy specimens can be obtained by an instrument called a Tru-Cut® needle (which collects a very small core of tissue) or by surgical incision of the mass to obtain a small wedge of tissue.

  • Radiographs. X-rays of the chest or nearby area of bone is often recommended to determine if any metastasis has occurred.
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