Injection site sarcomas (ISSs), as the name implies, are tumors of the skin and subcutaneous tissue that develop in cats secondary to a previous injection. They are most often implicated with vaccinations, however they could develop secondary to any prior injection, including those related to administration of drugs or even microchips.
I dislike all forms of cancer, but if I were forced to pick the one I despise the most, ISS would rank among my most loathed. When a pet develops a devastating and deadly tumor, as a consequence of something its owner did to try to keep it healthy and free from disease, it’s more than a terrible or unfortunate set of circumstances.
An essential part of treatment for a cat with an ISS is a carefully planned first surgery designed to remove the tumor with very wide margins. The current recommendation is to measure a 5cm radius of tissue around the tumor, and use this as the “edge” of where surgery should be done.
With this radical surgery, tumor recurrence is dramatically reduced and, consequently, patient’s survival times are longer than expected, as compared with the typical outcomes of more conservative surgeries.
Despite the better outcome, this form of surgery is rarely initially pursued because either the person performing the surgery is unable or unwilling to perform this aggressive procedure, or owners are unwilling to subject their cats to this type of treatment.
More frequently, tumors are removed with narrower planned margins, leading to disappointing outcomes. Narrowly excised tumors are extremely likely to recur without additional local control in the form of radiation therapy (RT). Even with aggressive pre- or post-operative RT, a decent proportion of tumors will regrow.
ISSs also have a modest chance for spreading to distant sites in the body, including organs such as lungs and regional lymph nodes. Chemotherapy is offered to try to prevent or delay this process from occurring; however, results are inconclusive as to providing a clear-cut benefit.
The treatment of feline ISS has recently shifted gears towards capitalizing on the patient’s own immune system to fight the tumor cells by using a novel protocol entailing the administration of Interleukin-2 (IL-2). IL-2 is a special type of protein that regulates the activity of white blood cells as part of an immune response.
The National Cancer Institute defines interleukin-2 (IL-2) as a biological response modifier, which is a substance that can improve the body's natural response to infection and disease. IL-2 stimulates the growth of disease-fighting blood cells in the immune system.
Kevin Whelan, Merial’s Technical Manager, explains the mechanism of action of how IL-2 works:
Following injection around the tumor surgery site, the recombinant canarypox vector virus enters the cat’s cells, which then produce interleukin-2. The presence of this cytokine stimulates an anti-tumor immune response by a variety of mechanisms, including the induction of T-lymphocytes and natural killer cells.
There is limited data regarding the efficacy of IL-2 for treating ISS in cats. One study showed a significantly longer time to tumor regrowth in cats treated with surgery, superficial radiation therapy, and IL-2 as compared to a reference group of cats treated with surgery and radiation therapy alone. This same study showed cats receiving IL-2 had a significant reduction of the risk of tumor relapse by 56% at one year and 65% at two years after initial treatment compared to cats not receiving IL-2.
I have no personal experience using the IL-2 immunotherapy, but I am always encouraged to try innovative anti-cancer treatments, especially for those diseases where options can be limited and outcomes can be poor.
I’ll admit it’s hard to talk to an owner about giving their cat a series of injections as a treatment for a tumor we believe was caused by an injection. It’s also difficult to discuss because the IL-2 treatment is manufactured by the same company that makes vaccines, the very substances that are implicated in the tumor formation in the first place.
Those issues aside, I think this represents an exciting new therapeutic for an otherwise devastating disease. I look forward to what the data will reveal regarding its success and implementing it in my clinic in the near future.
Dr. Joanne Intile
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