Radiation Therapy
Basic Information 
Overview and Goal
• Used to treat tumors that cannot be adequately treated with surgery.
• Primary effect on cancer cells is to damage the DNA and cause cell death when the cells try to divide.
• Radiation can treat visible tumor mass as well as surrounding areas affected microscopically by tumor so that areas a surgeon may not be able to remove may also be treated.
• Does not selectively seek cancer cells, but damage to normal tissue is reduced by fractionating the dose of radiation into many small doses or spreading the dose out over time through slow, constant delivery of dose. This allows normal tissues time to repair, repopulate, and survive the radiation insult.
• The total dose of radiation used is the dose needed to kill the tumor. This also determines the severity of the early effects that occur after irradiation. These early effects occur in rapidly dividing tissues, including epithelial surfaces of skin and mucosa.
• The fraction size is chosen based on the normal tissues being treated; some tissues will not tolerate large doses of radiation administered rapidly. These late-responding tissues, such as bone, muscle, and nervous tissue, divide slowly. They are intolerant of large doses per fraction. Adverse effects in these tissues can take years to appear.
• The period over which the radiation is delivered ideally is chosen based on the speed at which the tumor is growing.
• Total dose, fraction size, and total time in treatment must also take into account the increased difficulty to anesthetize and position horses. Therefore the number of fractions is often limited compared with smaller animal patients.
• Damage in normal tissues that do not contain tumor is also limited by delivering the dose of radiation as precisely as possible into the tumor. This requires treatment volumes that are as small as possible and shaped to match the tumor and its potential margins. This is termed treatment planning.
Indications
• In equine patients, external tumors and those of the extremities or head are easiest to position under the therapy beam or locally implant with radioactive seeds.
• Tumors need to have low metastatic potential to be good candidates for radiation therapy.
• Sarcoids and squamous cell carcinomas have been most commonly treated in the past.
• Localized low-grade lymphoma, sinonasal tumors, and malignant melanomas can also be treated.
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