Vaccine Associated Sarcomas

Oncogenesis is a multistep process. Three classic steps are considered:

• Initiation, in which a heritable cellular change

  Promotion, in which this change is "fixed" (unable to be repaired). This usually occurs after cellular division

  Progression in which a visible tumor appears

It is thought that repeated trauma and/or inflammation can act as a promoting step. The adjuvants in vaccines are designed to stimulate the immune system and thus induce inflammation. This appears to be the association between the tumor and the vaccine.

• The incidence of vaccine associated sarcomas is unknown. Some estimate it to be as high as 1/1000 vaccines. Others as low as 1/100,000. The truth probably lies somewhere in between

  These tumors may appear weeks to years post vaccine, and other trauma can be associated with them (subcutaneous fluids, etc)

  It is important to keep in mind that the adjuvant may travel long distance (long on a microscopic level) within macrophages and thus the tumors may not always appear in "classic" vaccine locations

  When faced with a cat with a post vaccinal lump that is small enough that it is not obviously a tumor, the best first step is aspiration cytology. While cytology is not conclusive, it is useful in determining whether to consider an aggressive resection. If the cytology is suggestive of inflammation, the owner should be told that the mass should be decreasing in size over the next several weeks. If it does not, re aspirate or a biopsy should be considered.

  When performing an aspirate or biopsy, care should be taken to avoid spreading tumor into tissue planes. Excisional biopsy may not be the best option, as all areas that are involved in such a procedure must be excised when definitive surgery is performed. Thus an excisional biopsy may result in a more aggressive surgery being necessary. When performing a needle aspirate or biopsy, the technique for an aggressive surgery should be considered as all needle tracts should be excised.

  Although these are usually fibrosarcoma, osteosarcoma, chondrosarcoma, and other soft tissue sarcomas may be seen histologically.

Treatment--Surgery

Aggressive surgery usually the best first step, although some patients may benefit from pre operative radiation (see below). Wide (3 cm) margins on the visible tumor is recommend. This may require partial scapulectomy and/or excision of the dorsal spinous processes. Cats tolerate these procedures very well, and should be seriously considered if a complete surgical excision is the goal. Ideally, this aggressive surgery should be the first surgical procedure the patient undergoes. If a less aggressive procedure has been attempted, and the client’s goal is to attempt a surgical cure, the aggressive surgery should take place within weeks of the initial procedure. "Shelling out" the tumor will inevitably leave microscopic residual disease, and should be considered cytoreductive and not curative. 

With conservative surgery, recurrence is very common. Typically recurrence occurs in 6-12 months.

Treatment--Post operative radiation

• Although sarcomas have been traditionally considered radioresistant, these reports are of conservative protocols used on animals with bulky disease. When higher total dosages are used in patients with microscopic residual disease, local control rates are fairly high.

As with surgery, a liberal margin around the tumor site (surgery site) is required. The entire scar must be treated. Radiotherapy planning may be facilitated by using radio opaque markers (steel suture, hemaclips) in the deep surgical margin. This allows the radiotherapist to identify the areas that are most likely to have microscopic residual disease.

Some institutions are adding doxorubicin or carboplatin chemotherapy to radiotherapy protocols. This seems to be well tolerated and needs further investigation.

• In one study done on cats who had between 1 and 10 surgeries prior to undergoing radiation, approximately 30% of remained free of disease for 2 years or longer. Those cats who were treated after their first surgery were more likely to fall in to this category. For those cats that did develop recurrence, the median disease free interval was approximately 18 months. 25% of the cats that did develop recurrence were successfully treated with conservative surgery and remained free of disease for 2 years or longer.

Treatment--Pre operative radiation

• In cases in which attempted resection may result in a very large incision, and especially if this incision may extend over the thorax, pre operative radiation should be considered. Radiation has the potential to kill the tumor cells that are on the edge of the tumor. These are the cells that most often are missed at surgery as they are not grossly detectable. The visible tumor will not likely shrink, so the owners should be warned that they may see no change during radiation therapy. Surgery is then planned for 2-3 weeks after radiation.

  This technique may be preferred to post operative radiation in cats with large tumors, but is not likely to make an unresectable tumor resectable. What it does is decrease the probability that microscopic residual disease will still be present if 3 cm margins are not attainable. This technique should seriously be considered when the cat has rapid recurrence after an initial surgery.

Treatment--Chemotherapy

• Chemotherapy is not the treatment of choice, however some cats may respond. It is an appropriate consideration for cats that have unresectable tumors, and when radiation is not an option (for financial or potential toxicity reasons). If possible, chemotherapy should be done after debulking surgery.

  Recent data reports a 40% overall response rate (complete responses were unusual) with a protocol containing doxorubicin and cyclophosphamide:

  Doxorubicin 1 mg/kg IV q 3 wks

  Cyclophosphamide 25 mg PO days 1 & 4

  The major toxicity of this protocol is anorexia. If it is severe, the cyclophosphamide should be reduced or discontinued. Myelosuppression may also be seen

The American Association of Feline Practitioners recommends that the following vaccinations should be given in the following sites:

• FVRCP right shoulder

  FeLV left hind leg (as low as possible)

  Rabies right hind leg (as low as possible)

  This standard procedure accomplishes two goals: First, it decreases the amount of adjuvant that is present at any one site; and second, it allows the profession to begin tracking the vaccines that appear to be associated with these tumors. It is also important to record site of vaccines, brand, and lot numbers and to help client keep accurate vaccine records to avoid over vaccination

To return to Sylvia's Cyber Kitty Condo just scratch her banner below.....

Or to visit the Veterinary Information Network homepage ...