Radiation dose, procedure type
For tumor management, the radiation dose is determined by radiosensitivity and the tolerance of nearby normal tissues. Connective,
osseous, muscular and nerve tissues are most tolerant to the early effects of radiation injury; the ovaries, testes and hematopoietic
and lymphoid tissues are the most vulnerable. Glandular tissue and squamous epithelium are intermediary in their sensitivity.
"When fractionated radiation is used, a total dose of radiation is chosen based on what is needed to kill the tumor," says
Fidel. "A time interval is chosen based on the tumor and the normal surrounding tissue, and a fraction size is chosen based
on the tissue in the field with the least ability to cope with large doses of radiation."
Radiation is measured in Grays (Gy), which vary with the type of treatment. Fraction size for curative treatments usually
is 3 to 4 Gy. For palliative treatments, the dose is often 6 to 10 Gy.
Technique variation is based on tissue type (soft tissue vs. bone), the tumor's location (superficial vs. deep or inaccessible)
and tumor size. The procedure is either teletherapy, in which the source of radiation is some distance (80 to 100 cm) from
the target tissue, or brachytherapy, in which sealed radioactive sources are applied directly to the area of treatment.
Teletherapy allows for delivery of radiation to large and deep-seated tumors. Unfortunately, it also exposes surrounding normal
tissue within the delivery path. With brachytherapy, high doses of radiation can be delivered directly to the tumor, which
limits the dose to affecting adjacent normal tissues.
Although not commonly used in horses, teletherapy uses machines that emit high-energy x-rays or gamma rays, with treatment
units in the megavoltage range (> 1 million eV). The high energy allows for precise treatment of large and deep-seated tumors,
especially bone or soft tissue-invading bone tumors. Low-energy units are not suited for horses since they produce limited
tissue penetration and may induce complications such as skin and bone necrosis. The tumor's size, depth and anatomical location
direct the teletherapy treatment plan. Precise, high-radiation dosing must be accurately positioned to attack the cancer with
minimal damage to normal cells. To accomplish this, the horse must be anesthetized.
Brachytherapy is better suited for use in horses because a high radiation dose can be delivered precisely and safely during
a short duration. It's best for soft-tissue tumors. It cannot be used for bone tumors or tumors invading bone because of the
limitations of implantation. Brachytherapy is most suitable for squamous cell carcinoma, sarcoid and soft-tissue tumors of
the head, distal extremities and genitalia.
Iridium-192 and iodine-125 are commonly used. They emit gamma rays in the form of "seeds" or wire that are implanted under
local or general anesthesia. It's best to use temporary implants that are left within the tumor for a specified time and removed
once the prescribed radiation dose has been administered.
With brachytherapy, "sources generally deliver a low-dose rate, but because they're left in place and exposure is constant,
a relatively high total dose of radiation is delivered over a short time (i.e., six days to two weeks, depending on the source
and its activity)," Fidel says.
Although brachytherapy is highly successful, it's not without complications and poses a hazard to equine (and human) patients,
says Fidel. "For eyelid squamous cell carcinoma, the radiation may affect the eye tissue, which cannot be shielded during
treatment," she says. "Also, equine patients implanted with radiation seeds may rub the tumor, dislodging the seeds and contaminating
Brachytherapy may be used in combination with surgery. For some tumors, debulking is helpful before radiation, but for others,
this process can create additional problems. It is critical to determine the extent of the tumor and tumor type before surgery.
Treatment options outlined
Clinicians at The Ohio State University (OSU) Veterinary Medical Center radiation therapy facility treat horses with a linear
accelerator. Brachytherapy is not available. During the past five to seven years, the center has treated about 40 horses.
"I think radiation therapy is most useful for the management of head and nasal tumors in horses," says Eric Green, DVM, Dipl.
ACVR (radiology and radiation oncology), clinical associate professor in the Department of Veterinary Clinical Sciences.
OSU clinic officials also use radiation therapy to treat sarcoids and squamous cell carcinomas—at least for those they can
get to with the limitations of the machine, which is generally the head, extremities and some of the neck. "I think it plays
a very useful role in the management of these tumors, often after surgery in many cases," Green says. "In my experience, we've
treated a lot of sarcoids that have been refractory to all other forms of therapy with great success."
Most cases treated at the OSU facility have been quite successful. "Unfortunately, there is very little literature to determine
what to expect in a lot of cases," Green says. "We're extrapolating a lot of the information we have from dogs, cats and people.
I think we might have better responses in horses than one might expect for a dog or a cat, as the tumors may behave somewhat
differently to the radiation therapy in horses."
Fidel notes that at Washington State University (WSU), squamous cell carcinomas are most commonly treated. For eyelid squamous
cell carcinomas, radiation therapy works quite well, as opposed to genital squamous cell carcinomas that are harder to treat,
though it is possible.
Officials at WSU do a novel protocol compared with other facilities. Horses are treated twice a day. "Radiation therapy works
very well," says Fidel, "and I often think it's sort of foolish that people don't consider the option a little sooner. Some
horses have had a variety of treatments, each of which last only for a short time. But a horse is going to live for 20 years,
so why don't you cure the problem with radiation therapy?"
Regardless of success rates, there are limitations, especially dictated by cost and the small number of available treatment
centers with proper radiation therapy equipment. There are only six locations across the country where practitioners can send
their patients for radiation therapy. Keeping a copy of the contact information for those equine radiation therapy centers
"As an oncologist, we always tell people to just call and we'll discuss the case and let them know the feasibility of doing
the procedure," says Fidel. "If you think you have a horse that's a candidate for radiation therapy, you should not only talk
to one of the radiation therapy facilities, but also send photographs and digital images of the tumor area to assist the radiation
oncologist in making the proper determination. Almost immediately, we can give an idea of whether it's treatable and what
side effects there may be, etc."
At WSU, oncologists consult with surgeons because not every eyelid squamous cell carcinoma is a good candidate. "If it's a
fairly localized tumor, the horse has a fair amount of value and the horse is going to live several more years, (owners) should
seriously consider the treatment," Fidel says. "It's curative. It makes the horse not just better for a short while—it can
cure the horse."