Radiation Therapy in Horses: Something to Consider
ACVIM 2008
Janean L. Fidel, MS, DVM, DACVR (Radiation Oncology), DACVIM (Oncology)
Pullman, WA, USA


Cancer may not be as common in equines as it is in our small animal population, but it does occur. In small animal oncology, cancer therapy is increasingly in demand and newer and better therapies are available. Radiation therapy is often used to control tumors locally and the radiation itself is being given in ever more sophisticated ways. The sheer size of horses can make radiation therapy seem impossible; however, it is a valuable tool for the treatment of cancer, and should not be over looked for the treatment of horses.


Radiation therapy works primarily by deposition of energy on or near the DNA of cells. Because the DNA is damaged, cells die when they try to divide. Although radiation seems like magic, it isn't as selective as we would like. Normal and cancer cells are killed, and only a constant proportion of cells are killed with each dose. If single large doses of radiation are given it may be possible to kill a tumor but normal tissues would be severely damaged. Better survival of normal tissue is achieved when radiation is given over a prolonged time, or is broken into many small doses of therapy. Factors affecting the cells sensitivity to radiation therapy are their ability to repair, their ability to repopulate, where they are in their cell cycle, and the presence of oxygen. Theoretically, these parameters can change over time or after a dose of radiation is given and a portion of the cells are destroyed, and they are often referred as the 4 R's- or Repair, Repopulation, Redistribution, and Re-oxygenation. These 4- R's form the basis for most fractionated radiation protocols, which involve delivery of several small fractions of the total dose of radiation--instead of one large dose. The attempted goal is to kill the maximum number of tumor cells and yet allow time for repair and repopulation to occur in the normal cells so that the normal cell population survives.

A total dose of radiation is chosen based on what is needed to kill the tumor, a time interval is chosen based on the tumor and the normal surrounding tissues, and the fraction size is chosen based on the tissue in the field that has the least ability to cope with large doses of radiation. These are late responding tissues, or those with the least capacity for repair such as nervous tissue, bone, and the fibrous layers in skin. Typical fractionation schemes in small animals are 2-3 Gy fractions, daily M-F, to a total of 18-20 fractions. Variations are sometimes made to treat tumors that repopulate too quickly (e.g., oral squamous cell carcinomas) where the overall treatment time is shortened by treating twice a day. Also tumors deemed "incurable" due to size (the surgeons didn't want to touch it) or metastatic potential (melanomas) can be treated with less aggressive, more coarsely fractionated protocols (6-8 Gy fractions 3-5 times) because the patient is not expected to live long enough to experience the "late" side effects of radiation.

When considering the tumor as a whole as a candidate for radiation therapy, other factors come in to play--the number of cells that needs to be treated/the size of the tumor, sensitivity of the tumor cells, and most importantly,--the sensitivity of the surrounding tissues to effects of radiation. Often the best plan is to decrease the tumor size and actually increase the sensitivity of the tumor cells by removing as much as possible surgically. Generally, surgery is planned with the attempt to remove the entire tumor, or at least all that is visible. Radiation is then expected to only clean up microscopic dirty margins. For some tumor types, getting rid of whatever you can, or "debulking," can be helpful prior to radiation while for others the debulking process can create additional problems. This is why determination of tumor type and extent is critical prior to surgery.

The goal of radiation treatment planning is to deliver the highest possible dose to tumor, and the lowest dose to normal surrounding tissue. This may seem easy--but at times it can be quite difficult and even impossible. Surrounding tissue sensitivity is something that usually can not be changed, and it can at times be the dose limiting toxicity in the treatment of the tumor. Generally, doses have to be adapted to spare neurologic tissue and bone (fraction size and total dose), but eyes, the oral cavity, and other structures can also limit radiation's ability to cure. Despite the best attempts at sparing normal tissues, there are side effects to radiation therapy. These can generally be divided into early and late effects. Early effects happen within 3 months post-therapy, are expected, and will get better. Early effects involve tissues that are rapidly dividing. They include hair loss, irritation of the skin caused by the cornified outer layer of the skin being absent for a time, mucositis, and conjunctivitis. Symptomatic therapy and patience is generally the best treatment for early side effects, and care must be taken not to damage the tissues further (animal scratching, or human scrubbing). Late effects occur months to years after radiation and will not get better. Acceptable side late effects include alopecia and hyperpigmentation of the skin, and cataract formation. Less acceptable effects would be nervous tissue atrophy or necrosis, bone necrosis, and skin fibrosis. These are serious side effects that may mean the treatment itself was done improperly.

Types of Radiation and Use in Equines

Brachytherapy--Or Short Distance Radiation Therapy

This involves placement of radiation very close to the tumor or most often within the tumor itself. Interstitial therapy uses small seeds of radioactive material implanted into a tumor. In many ways this is the ideal way to treat many superficial tumors as the treatment is confined to the tumor area. Also sources generally deliver a low dose rate, but because they are left in place and exposure is constant, a relatively high total dose of radiation is delivered over a short time period. Implantation can be performed under a local anesthesia or a single anesthetic episode. Many types of interstitial seeds have been used in horses in the past with the main variable being the radioactive source and its half life. Gold 198 (T1/2-2.7 days), Radon 222 (T1/2-3.83 days), Iridium 192 (T1/2-74.2 days), Iodine 125 (T1/2-60.2 days), and even Cesium 137 (T1/2-30 years) and Cobalt 60 (T1/2-5.26 years), have all been used in equine radiation therapy.1-11 Implants are left in place temporarily or permanently. Permanently placed seeds are left in place and the dose is delivered slowly over time as the radioactive source decays. Temporary implants are meant to be left in place until the desired dose of radiation has been delivered and then removed. Seeds are surgically implanted using pre-placed nylon catheters into which the radioactive sources are inserted using long handled forceps. Handling of the seeds and handling of the horses with implants are all potential times for humans to be exposed to radiation, and some states and facilities no longer allow interstitial therapy in equines. For temporary seeds the implants must also be removed, again a potential source of human exposure. An alternative is to use a "remode afterloading" device with a high activity source (Iridium 192). After placement of catheters a single source controlled by a computer enters each catheter for prescribed distance and time allowing treatment of the entire tumor. This form of brachytherapy delivers a fairly high dose rate in a short time and the treatments must be fractionated into more than one treatment.

Use of interstitial implants in equines tumors has been highly successful for the treatment of sarcoids and cutaneous squamous cell carcinoma (SCC). Early studies with Cesium or Cobalt and periocular SCC (1978) showed good results with two year tumor control rates of 68 %.1 Subsequent studies have shown tumor control rates at 2 years of 74-81%. Sarcoids have been treated with slightly higher success rates between 87-100%. Important factors in determining the success of therapy include the total dose of radiation delivered and the size of the tumor. Smaller tumors will be better controlled which often necessitates a surgical debulking be performed first.

Although brachytherapy is highly successful it is not without complications and hazards to both the patient and the humans involved in the care of the horse. Dose of radiation is determined by time of decay of the source or time left in place in the case of temporary implants, also--proximity of one source to another. With precise planning the dose can be evenly distributed and hot spots can be avoided. Treatment around eyes will cause radiation of the eye itself because the eye cannot be shielded while the treatment is ongoing. Horses are to remain isolated--but there will still be human contact. Also it can not be guaranteed a horse with seeds in place will not rub at the tumor and dislodge seeds implanted temporarily.

Plesiotherapy--Close to, or Very Short Distance Radiation Therapy

This form of radiation is usually used to describe treatment with Strontium 90. This beta emitting source is hand held and can treat an area of about 5-8 mm. Because it is beta radiation the maximum penetration into tissue is only 3 mm, with 60% of the dose being delivered within the first 1 mm. A high surface dose of radiation (20-25 Gy) can be given because the dose is extremely localized. Use of Strontium 90 for radiation therapy is also highly successful but is limited to small superficial tumors and often this is a post surgical treatment.4,9,12 Time needed for a single treatment site is around 10-15 minutes, and multiple contiguous sites can be treated to treat a larger area. Over time the sources decay, and lower radiation doses rates are delivered such that treatments require more time. Generally the horse must be fully anesthetized or heavily sedated to accomplish therapy.

Teletherapy--Therapy at a Distance

Teletherapy involves use of an external beam of radiation to treat a tumor at any depth. At one time external beam therapy was similar to interstitial therapy in that a radioactive source was used (Cesium or Cobalt) to generate gamma rays, or a photon beam. These days most external beam radiation is delivered using a linear accelerator which often has the capability of producing two distinct forms of radiation. Electrons themselves are used as therapy beams and this allows for the precise treatment of superficial treatments. Accelerated electrons striking a target generate photon beams much like a diagnostic x-ray unit, except the energy is much higher (at least 6 MeV). These high energy beams are used for penetrating more deeply into tissues. High dose rate therapy is delivered to a localized field; however surrounding tissues are treated as well. Therapy is nearly always fractionated to bring about a better tumor kill and less side effects to surrounding tissues. Linear accelerators are costly pieces of equipment and rooms with maximal shielding are required to house them. Patients must remain absolutely still to be sure the treatment is being given in the correct location as the beam is stationary and does not move with the patients. Also no one is in the room when therapy is occurring. For animal patients this necessitates anesthesia.

For equines many biases against teletherapy have long existed. The need for repeated anesthesic episodes was often a big deterrent. Even today, facilities that can irradiate horses will often use fractionated protocols involving much fewer fractions of therapy than would be used for small animals. The facilities themselves must be built with equine treatment in mind. Small animals can be treated on a treatment couch designed for humans, and they are often anesthetized in the same room. Horses need their own treatment tables, anesthesia is induced at another location requiring transportation of the horse into the treatment room. The Linear accelerator itself can treat a horse; it is everything around the machine that needs modification. Because of these concerns it seems as if a bias has also built up with the equine world against considering radiation therapy as potential treatment for equine tumors, despite the good results achieved with other forms of radiation therapy used. Outcomes for horses with SCC of the eyelid and peri-ocular structures were significantly better if treated with adjuvant radiation therapy versus those that were not, with recurrence rates of only 11.9% versus 44.1%.9

External beam radiation has been used on a limited basis to treat deep seated tumors in equines. Reports have been published of treating; a mandibular ossifying fibroma where 2 years later, the tumor had not returned 13; SCC of the nasal cavity and paranasal sinuses in three horses 14 where survivals were 6, 3.5, and 2.5 years; and a recurrent ossifying fibroma in the paranasal sinuses of a horse where tumor was controlled for > 6 years.15 Unpublished cases are likely numerous, but still at much lower numbers than what is currently being treated in small animal oncology. A variety of tumors have been treated at UC Davis including head and neck SCC, osteomas, fibromas and ameloblastomas of the oral cavity, melanomas, nasal tumors, and extremity SCC, and other institutions including WSU (see list at end). Radiation outcomes have been uniformly successful and side effects are generally minimal. And such good results have often been attained with fairly coarsely fractionated protocols meaning fewer and larger dose fractions than would be thought of as ideal for small animals or humans.

At WSU the Linear accelerator vault was built with treatment of horses in mind. A variety of equine tumors have been treated over the past 10 years but generally the bias against multiple anesthesic episodes has existed here as well. More recently when considering treatment of cutaneous SCC, the question arose as to which was easier with a horse--hold them off food every other day for anesthesia 9-10 times over a 3-4 week period (typical coarse fractionation protocols would require this) or anesthetize them twice daily for 5 days and be done (an accelerated protocol). Accelerated radiation protocols as stated earlier are aimed at treatment of rapidly growing tumors, such that tumors have little time to recover between treatments. Twice daily treatments have been used for treatment of feline SCC, and human SCC. Also the shortened treatment time would more closely approximate therapy delivered using interstitial radiation. Our protocol became 2 treatments daily divided by a space of 6 hours, delivered over a 7 day period because treatments were divided over a weekend (cats are generally treated in a total of 5 days). Three horses have been treated with this rapid protocol and none had problems related to anesthesia other than the largest of the 3 having slow recoveries forcing some single treatment days and a total protocol of 9 days. Two horses with SCC had complete resolution and one sarcoid resolved for roughly one year but then began to reoccur. This protocol is not ideal for every tumor or location, but it does highlight the fact that the general bias against multiple anesthesic episodes, in close succession, in a horse, may be unfounded.

Tumors one should consider as candidates for RT are numerous, and in small animal medicine there is little we won't at least attempt to palliate with radiation. One of the few requirements is that the tumor has not metastasized or in general has no tendency to metastasize. Location can make treatment easier in that extremities and head are much easier to position but other areas can be treated as well as long as the body part fits under the beam. Melanomas and sarcoids can be treated with large fraction radiation (only 3-4 doses of 6-8 Gy) quite effectively. Squamous cell carcinomas obviously respond well to a variety of protocols. Lymphosarcoma in a single location will respond exquisitely and rapidly to RT. Cost can be a deterrent but if owners can be offered a treatment that will give their horse an additional 2-10 years longer, without loss of a body part such as an eye, they would probably be willing to spend the $2,000.00-6,000.00 that it will cost to have their horse irradiated. Travel can also be problem, but facilities willing to treat horses are scattered across the US. Perhaps the biggest limiting factor to equine radiation is the mind set of clinicians.

Facilities Offering Equine Radiation in North America

 Auburn, Alabama (brachytherapy, teletherapy), Dr. Bill Brawner, phone 334-884-5045, email brawner@vetmed.auburn.edu

 Missouri (brachytherapy, teletherapy, radio-isotopes)--Dr. Jimmy Lattimer, phone 573-882-7821, email lattimerj@missouri.edu

 Ohio State University (teletherapy)--Dr. Eric Green, phone 614-292-6661, email green.689@osu.edu

 Texas A&M (brachytherapy, teletherapy)--Dr. Michael Walker, phone 979-845-9081, email mwalker@cvm.tamu.edu

 U.C. Davis (brachytherapy, teletherapy)--Alain Theon
Washington State University (teletherapy)--Janean Fidel, phone 509- 335- 0711, email jfidel@vetmed.wsu.edu


1.  Gavin P, EL G. Interstitial radiation therapy of equine squamous cell carcinomas. Veterinary Radiology and Ultrasound 1978;19:138-140.

2.  Wyn-Jones G. Treatment of periocular tumours of horses using radioactive gold198 grains. Equine Vet J 1979;11:3-10.

3.  Frauenfelder HC, Blevins WE, Page EH. Rn for treatment of periocular fibrous connective tissue sarcomas in the horse. J Am Vet Med Assoc 1982;180:310-2.

4.  Frauenfelder HC, Blevins WE, Page EH. 90Sr for treatment of periocular squamous cell carcinoma in the horse. J Am Vet Med Assoc 1982;180:307-9.

5.  Wyn-Jones G. Treatment of equine cutaneous neoplasia by radiotherapy using iridium 192 linear sources. Equine Vet J 1983;15:361-5.

6.  Wilkie DA, Burt JK. Combined treatment of ocular squamous cell carcinoma in a horse, using radiofrequency hyperthermia and interstitial 198Au implants. J Am Vet Med Assoc 1990;196:1831-3.

7.  McConaghy FF, Davis RE, Reppas GP, et al. Management of equine sarcoids: 1975-93. N Z Vet J 1994;42:180-4.

8.  Theon AP, Pascoe JR. Iridium-192 interstitial brachytherapy for equine periocular tumours: treatment results and prognostic factors in 115 horses. Equine Vet J 1995;27:117-21.

9.  Mosunic CB, Moore PA, Carmicheal KP, et al. Effects of treatment with and without adjuvant radiation therapy on recurrence of ocular and adnexal squamous cell carcinoma in horses: 157 cases (1985-2002). J Am Vet Med Assoc 2004;225:1733-8.

10. Byam-Cook KL, Henson FM, Slater JD. Treatment of periocular and non-ocular sarcoids in 18 horses by interstitial brachytherapy with iridium-192. Vet Rec 200 6;159:337-41.

11. Ellis DR. Treatment of squamous cell carcinoma in a horse. Vet Rec 2006;159:462-3.

12. Plummer CE, Smith S, Andrew SE, et al. Combined keratectomy, strontium-90 irradiation and permanent bulbar conjunctival grafts for corneolimbal squamous cell carcinomas in horses (1990-2002): 38 horses. Vet Ophthalmol 2007;10:37-42.

13. Robbins SC, Arighi M, Ottewell G. The use of megavoltage radiation to treat juvenile mandibular ossifying fibroma in a horse. Can Vet J 1996;37:683-4.

14. Walker MA, Schumacher J, Schmitz DG, et al. Cobalt 60 radiotherapy for treatment of squamous cell carcinoma of the nasal cavity and paranasal sinuses in three horses. J Am Vet Med Assoc 1998;212:848-51.

15. Orsini JA, Baird DK, Ruggles AJ. Radiotherapy of a recurrent ossifying fibroma in the paranasal sinuses of a horse. J Am Vet Med Assoc 2004;224:1483-6, 145.

Speaker Information
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Janean Fidel, MS, DVM, DACVR (Radiation Oncology), DACVIM (Oncology)
Washington State University
Pullman, WA