Radiation was first used on horses in 1906. Today, finer, more sophisticated therapeutic techniques, procedures and equipment
can be used to treat the relatively uncommon cancerous tumors in horses.
What's important to a successful outcome is delivery of the radiation dose to eradicate the tumor while sparing surrounding
healthy tissues. Fortunately for horses (and people), healthy tissue is more resilient, and current individualized techniques
can deliver radiation to the tumor site with fairly good precision. Prognosis, of course, depends on tumor size, type and
location.
"The large size of equine patients can make radiation therapy difficult, but it's a valuable tool for treating cancer and
should not be overlooked when treating horses," says Janean Fidel, BS, MS, DVM, Dipl. ACVIM (medical oncology), Dipl. ACVR
(radiation oncology), associate professor at Washington State University's College of Veterinary Medicine. The basis for the
procedure entails damaging tumor tissues with ionizing radiation through the use of electromagnetic X-rays and gamma ray photons
or corpuscular radiation (streams of electrons).
Radiation therapy's effectiveness is accomplished by destroying the cellular DNA, damaging one of the two strands, thereby
rendering cells unable to divide so that they die attempting mitosis. Therefore, a tumor's growth rate plays a role in its
demise. Only a portion of the cancer cells are killed with each dose. Although radiation damages both cancer and normal cells,
the healthy cells can repair to a greater extent and survive by accelerating their division of the surviving cells, while
cancer cells are more vulnerable. Healthy cells' ability to survive while cancer cells succumb is noted as the selective effect of irradiation. Furthermore, "Survival of normal tissue can be improved when radiation is administered over a prolonged period of time or
fractionated into many small doses," says Fidel.
Equine radiation therapy can attack cancerous tissue, with radiation energy directed on the tumor cells in the area being
treated. Radiation is ineffective for treating distant metastasized cells. The therapy is best indicated for solid tumors
confined to a limited anatomical area and for those that are locally invasive. The most common tumors that are curable are,
in descending order, squamous cell carcinoma or papilloma, sarcoid, soft tissue sarcoma and melanoma.
Radiation therapy is tailored to the particular tumor. "For deep-seated tumors that require imaging for assessment, computerized
treatment plans based on the images are most accurate for predicting what the treatment will deliver to the tumor and normal
tissue," says Fidel. Tumors of the same histologic type may respond differently depending on where they're located.
In addition to radiation therapy, initial accompanying surgery to debulk and minimize tumor size is important to a good outcome.
Radiation before surgery improves resectability by reducing the tumor size and sterilizing tumor margins, thus making for
easier surgical removal. Or radiation may be used postsurgically to eradicate tumor cells left behind after an incomplete
excision.
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