Cancer in a Horse

Last Modified: November 13, 2005


Dear OncoLink "Ask The Experts,"
My 4 year-old mare has just had a growth removed from her throatlatch. The pathology report showed the growth as "spindle cell sarcoma". The growth was apparently benign. My local vet has not dealt with this before. Any suggestions?


Lili Duda, VMD, Section Editor of the OncoLink Veterinary Oncology Menu, responds:

There is relatively little information on cancer in horses. Assuming that this tumor will behave similarly in the horse as it does in the dog, it would fall into the category of "low-grade soft tissue sarcoma" (although there is no descriptive pathology report provided, the fact that the owner reports that the tumor is "benign" suggests a low-grade tumor). These tumors are locally invasive into surrounding tissues, but are unlikely to metastasize, or spread elsewhere, in the body. The best treatment option, when possible, is wide surgical excision. The entire biopsy specimen should be sent to a pathologist, and the surgical margins (preferably inked or otherwise marked by the surgeon) should be evaluated for completeness of the excision. If the surgical margins are "clean", or negative -- in other words, if it does not appear that cancer cells are present at the cut edges, indicating that residual tumor cells are unlikely to remain in the patient -- then no further treatment is indicated. However, the surgical site should be evaluated periodically for evidence of local recurrence.

If there is any suggestion that tumor cells remain behind, a larger surgery is recommended. If this is not possible, radiation therapy is a reasonable option, but has very limited availability for large animals. Alternatively, intralesional chemotherapy (injecting chemotherapy drugs directly into the area of the tumor) has been anecdotally reported, but there is little in the veterinary literature regarding this treatment option. Consultation with an equine medicine or surgery specialist is recommended for more information. These tumors may behave differently in the horse than they do in the dog. You may be able to find an equine specialist through one of the following links:

The veterinary Cancer Society has a helpful website, offering resources for pet owners, including a Find a specialist in your area.

The American College of Veterinary Internal Medicine has a website with a section for pet owners. They are currently adding a resource to find a specialist.

In addition, many local and state veterinary medical associations have information on specialists. The American Veterinary Medical Association has links to local organizations.

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