External Beam Radiation Therapy in Horses

Oncology in horses is an area of veterinary medicine which has been relatively neglected until recent years. Equine tumors commonly remain localized for prolonged periods before spreading but can cause serious problems locally. There is a need to develop successful methods of preventing recurrence and prolonging a good quality of life. External beam radiation is a cutting edge therapy for equine tumors particularly of the extremities (head and limbs) and skin. Treatment decisions always need to be made with full involvement of referring veterinarians, owners, and with considerations for the quality of life, possibility of success, and economics. There are many ways of treating equine tumors with surgical excision, local chemotherapy, cryotherapy, and laser assisted excision being the most common but recurrence remains a common problem. Systemic chemotherapy in horses is relatively rare due to the expense associated with many medications. At The Ohio State University Galbreath Equine Center there is a focus on equine oncology with collaborations between the surgeons, and radiation oncologists. Internal medicine and oncologists also consult on cases to provide the best possible care. The addition of a new CT machine is also crucial to the success of both surgery and radiation as it allows accurate identification of the structures involved in the tumor and delineates the radiation field.

In the last couple years there have been two areas of particular focus in equine oncology and potential applications of external beam radiation therapy; the treatment of skin tumors and the treatment of tumors involving the equine head.

Horses with tumors of the head range in age but often occur in older patients. A variety of tumors occur in the head of horses and include squamous cell carcinomas, ameloblastomas, fibrosarcomas, osteosarcomas, adenocarcinomas, and ossifying fibromas to name a few. They can occur in the sinuses, nasal passage, maxilla or mandible. Surgical excision is the current standard treatment at most referral surgical centers but tumors frequently recur as complete excision is difficult. Radiation treatment using external beam radiation has been used in horses in the past couple years in combination with surgical excision at the Galbreath Equine Center. The protocols used are tailored to each individual horse taking into consideration the tumor type and behavior, the economics, and expected results. Palliative and definitive radiation protocols can be designed. In order to receive external beam radiation the horses must undergo a short (average 15-20 minutes) general anesthetic. A young horse with a large fibrosarcoma in the rostral maxilla and an aged horse with an osteosarcoma encompassing the entire right nasal passage are just two examples of horses who have received post-operative radiation therapy successfully with no tumor recurrence. Both horses have returned to normal athleticism. The success rate has been very promising at preventing or delaying recurrence and prolonging a good quality of life. Importantly the treatments have been accomplished easily and the horses have an excellent quality of life during treatment. Side effects have been limited to white hair in the radiation field and mild, short-lived mucositis when mucocutaneous junctions are in the radiation field. No systemic illness, inappetance, or depression, have been noted.

Horses with skin tumors can be frustrating patients as these tumors are very slow to metastasize but can be locally very irritating, cosmetically unsightly, and interfere with function. The most common skin tumors are melanomas, sarcoids and squamous cell carcinomas. Sarcoids have been plaguing veterinarians with their tendency to recur for years and recently these have been given the option of surgical debulking followed by external beam radiation. Those sarcoids in difficult places such as the peri-ocular region are excellent candidates for radiation therapy. It has been established that sarcoids respond well to radiation as brachytherapy has reportedly been very successful in preventing recurrence. Brachytherapy is the implantation of radioactive beads or ribbons into the tumor. This has some drawbacks, however, as the horses are then radioactive for the duration of therapy and must be housed and treated in special circumstances that are laborious and expensive. External beam radiation can be done on an outpatient basis, as the horses are not radioactive at any time. The ease of administration and the lack of significant side effects are amongst the benefits of external beam radiation. Data is being accumulated on the long term response of sarcoids to external beam radiation therapy but early results are promising and it has been used successfully in sarcoids that have failed to respond to multiple other therapies.

External beam radiation therapy is a cutting edge adjunctive therapy which is being utilized to prolong and improve the quality of life in our equine patients. Questions regarding the use of radiation for equine oncology patients can be directed towards Dr. Eric Green, Dr. Margaret Mudge, or Dr. Shannon Reed (contact information below). The clinicians at OSU are dedicated to improving the options available for treatment and methods to improve our success for our equine patients.

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