Local Chemotherapy & Laser Hyperthermia for Treatment of Equine Skin Tumors
August 18, 2023
By Sarah Matyjaszek, D.V.M., DACVS-LA
Clinical Associate Professor of Large Animal Surgery
Large Animal Clinic, part of the Animal Health Institute
Midwestern University College of Veterinary Medicine
This article originally appeared in the August 2023 issue of Arizona Horse Connection magazine, page 28.
Equine skin cancers are very common, with tumors including sarcoids, squamous cell carcinoma, and dermal melanomas of gray horses. With the exception of squamous cell carcinoma, skin tumors do not metastasize to other areas of the body but can be locally aggressive with a high recurrence rate. Treatment depends on tumor type and location, as well as economics. Most treatments consist of surgical removal and/or some form of ancillary therapy aimed at killing any residual tumor cells. Ancillary therapies most often include local chemotherapy, cryotherapy, laser hyperthermia, photodynamic therapy, and radiation therapy. Fortunately, horses do not appear to suffer negative side effects from radiation or local chemotherapy. This article focuses on two specific treatments of equine skin tumors—local chemotherapy and laser hyperthermia.
Local chemotherapy involves either injection or implantation of a medication that prevents repair of DNA, leading to cancer cell death. The most commonly used medications are cisplatin or carboplatin, which are platinum-based. Platinum from beads diffuses 12-15 mm and lasts a month, while injections do not diffuse and last only two weeks. At a horse’s first visit, the tumor is surgically removed if possible. Treating less tumor mass always makes sense. Depending on the size of the site, a number of beads are implanted into small stab incisions in the area, which are then closed with an absorbable suture. In general, a series of three treatments is required for best results, with some tumors requiring additional treatments. Horses tolerate the procedure well. Since the beads were developed more than 20 years ago, hundreds of tumors have been successfully treated using them as primary or ancillary treatments.
Photodynamic therapy employs a photosensitizer drug instilled into a tumor. A specific wavelength or “color” of light interacts with the drug, generating heat and killing the cells. Dermal melanoma is a very common skin tumor of older grey horses. The lesions are dark, black, rounded, and nodular skin tumors (resembling charcoal on cut surface) and have a predilection for specific areas on the horse including the base of the tail, anus, lips, and parotid salivary gland. The black pigment imparts a natural photosensitizing property with laser light of a wavelength absorbed by dark pigment. Our Class 4 diode therapy laser produces such a wavelength. Laser light raises the tissue temperature higher than the cells can stand without creating an open wound. This process is called laser hyperthermia. Small dermal melanomas can be thoroughly heated by laser light energy and will die back.
Along with treating all tumors, the Large Animal Clinic is currently seeking equine patients with smaller (1 cm maximum) sarcoids and gray horse dermal melanomas. The company with which we developed the chemotherapy beads years ago is seeking FDA approval for that product, and we are assisting by treating cases to re-document laser hyperthermia efficacy on sarcoids. Although we know these techniques work, the FDA requires formal documentation for a series of current cases. We have received funding to cover treatment costs for a small number of appropriate cases, and strict adherence to the evaluation schedule is required. For more information and to find out if your horse may qualify/schedule an evaluation, please call the Large Animal Clinic at 623-806-7575 and ask to speak with Dr. Sarah Matyjaszek or Dr. Ken Sullins.
The information contained in this article is not intended to be a substitute for professional medical advice, diagnosis, care, or treatment. Always consult your veterinarian with any questions regarding any possible medical condition.
Sarah Matyjaszek, D.V.M., DACVS-LA, is a board-certified large animal surgeon. She earned her Doctor of Veterinary Medicine degree from Michigan State University and completed an internship and residency in large animal surgery. Dr. Matyjaszek serves as Clinical Associate Professor of Large Animal Surgery and supervises third-and fourth-year veterinary students in their clinical rotations at the Large Animal Center at the Midwestern University College of Veterinary Medicine Animal Health Institute in Glendale, Arizona.