A Hazard of Vaccinations

Vaccine-associated sarcomas are serious tumors that can be life-threatening. Know what to look for.

Fibrosarcomas are malignant tumors of connective tissue cells. They are solitary tumors of variable size, irregular or nodular in shape, and are the third most common skin tumor in the cat. In most cases the cause is unknown, but tumors found in young cats – or those that appear as multiple rather than solitary masses – may be caused the feline sarcoma virus (FeSv), a close relative of the feline leukemia virus (FeLV). Some fibrosarcomas appear at places on the body where vaccines have been given, and are referred to as vaccination-associated sarcomas.

They Occur Infrequently
How commonly vaccine-associated sarcomas occur is difficult to ascertain, partly due to the difficulty in obtaining an accurate history of exactly which vaccine was given in which location and at what time. Most studies indicate that they occur quite infrequently – approximately one to four per 10,000 vaccinated cats.

Nevertheless, a cause and effect relationship has been established, and the Vaccine-Associated Feline Sarcoma Task Force (VAFSTF) was formed to address the issue (see www.avma.org/vafstf).

Additionally, the American Association of Feline Practitioners has published vaccination guidelines in regards to frequency and recommended vaccination sites (see www.aafponline.org/).

The hardest part is trying to get really solid statistics, says Barbara Gores, DVM, with Southwest Veterinary Specialty Center in Tucson, Arizona, and a specialist in small animal surgery. The problem is trying to identify how many tumors are actually caused by a vaccine versus something else. We do know fibrosarcoma has a very high recurrence rate, but how best to treat it is the challenge. It seems that no matter what we do, the recurrence rates are still quite high. These tumors are very locally invasive, but have a very low distant metastatic rate (the tendency to appear in sites far removed from the original growth), so the challenge is to obtain local control of the tumor to prevent recurrence. 

The two most common areas of occurrence are between the shoulder blades and on the hind limbs, and it isnt limited to older cats. Most commonly, I see cats four to five years of age and older. Fibrosarcomas seem to cause minimal discomfort; a cat with a palpable or visibly good-sized mass can be running around apparently feeling fine, says Dr. Gores.

When a cat develops a lump, fibrosarcoma should be suspected, so it is important to first determine the tumor type. (This will determine the prognosis and treatment options.) A veterinarian might first perform a minimally invasive fine-needle aspirate to identify the cells via cytology.

The problem with sarcomas, as compared to other tumors, is that they tend not to shed cells very easily, so you may not get an accurate diagnosis. It may just look like inflammation, notes Dr. Gores. 

A preferable procedure – a core biopsy – may be performed, using a larger gauge needle with the cat under heavy sedation or anesthesia. An alternate more invasive option, depending on the location, and size of the tumor, is to obtain a tissue sample through a small incision. Its critical with biopsy procedures not to contaminate healthy tissue by dragging cancer cells through it, says Dr. Gores.

Aggressive Removal Important
The treatment of choice is excision, which is true for most tumors.  A basic general rule is if you can excise it with wide complete margins, thats absolutely the best way to go. Even if you cant excise it completely, youre better off to reduce the tumor volume because other treatments – radiation, chemotherapy, immunotherapy – have a much better efficacy rate if theyre working on microscopic tumor cells, says Dr. Gores. The first surgery is your best chance of excising all of the tumor. Be as aggressive as you can the first time because if you leave tumor behind, trying to go back and clean up a microscopic tumor later, not knowing exactly where to go, can be tricky.

For the procedure itself, Dr. Gores describes, because you need to be very aggressive, its important that the person doing the surgery is comfortable and familiar with surgical oncologic techniques. A general veterinarian who does not perform cancer surgeries routinely may be tentative about removing large amounts of tissue.

A large hole requiring closure may be left, and we may need to mobilize skin or do skin flaps, says Dr. Gores. General veterinarians may be less experienced with such reconstructive procedures. In the hind limbs, depending on the location of the tumor, we may suggest amputation as the first course of treatment, because getting complete tumor excision with wide and deep margins is impossible otherwise.

For tumors in other areas, because the incision is so large, the shaved area so wide, and the recurrence rate so high, I spend a lot of time educating the cats owner prior to surgery so that he or she can make an educated decision, says Dr. Gores. Current thinking is that fibrosarcomas can actually skip over zones of healthy tissue and send out little satellites, so even with aggressive surgery and post-operative adjuvant radiation therapy, recurrence is likely.

Prognosis is always guarded, but do not avoid vaccinations. The benefits far outweigh the possible risks, so discuss with your veterinarian an appropriate vaccination schedule for your cat.