Veterinary Radiation Therapy

OncoLink Team
Last Modified: August 24, 2018

Radiation therapy has become one of the most common forms of cancer treatment in veterinary oncology. It is used to treat localized disease. Radiation can be used in the management of cancers that cannot be treated successfully by surgery or chemotherapy alone. Typically, it is used after surgery when there are tumor cells remaining.  In some instances, radiation therapy may be used before surgery or chemotherapy to try to shrink a tumor down to a more manageable size.

How does radiation therapy work?

Radiation makes cells unable to replicate. The idea is to eliminate the cancer cell so that they can no longer divide and spread. Because both normal and cancer cells are affected, the radiation machine can be positioned in the specific area of the tumor. Radiation treatment is intended and designed to maximize tumor effect and minimize normal tissue effect. Maximizing tumor effect is one reason that radiation treatments are given as a series of many small doses, rather than a few large doses. 

What are the benefits of radiation therapy?

Radiation therapy can offer, in some instances, cure of the cancer. Even when a cure is not possible, radiation therapy can still bring some relief. Shrinking a large tumor with radiation therapy may improve a pet's quality of life by reducing pressure, bleeding, or pain. This is called palliative treatment.

Are there risks involved?

There are some risks involved with any type of cancer treatment. In addition to cancer cells, some normal cells will also be killed by the radiation. Some side effects may be apparent because of normal cells being killed (e.g. "radiation dermatitis"). Usually these side effects are outweighed by the benefits of killing cancer cells.

In addition, radiation therapy requires the animal to be perfectly still during treatment. Thus, anesthesia is necessary for each treatment. There is always a very slight risk associated with anesthesia.

How is the therapy given?

A linear accelerator directs a radiation beam toward the cancer and some normal tissues around it. The equipment is in many ways similar to equipment used for standard X-rays, except that the energy of the radiation beam is much higher and the exposure times are much longer. Treatment takes approximately 5-20 minutes. Sometimes CT or MRI images are used with a computerized treatment planning system to more carefully target a tumor and avoid critical normal structures near the tumor.

Radiation therapy differs per individual pet. A Radiation Oncologist will map out the best plan for your pet specifically. The Radiation Oncologist and their trained veterinary technician team will deliver the treatments. Treatments can be daily or once or twice a week. Hospitalization may be suggested, but often times your pet can be an outpatient. 

Is radiation therapy ever used in combination with chemotherapy or surgery?

Yes. In situations where it is unlikely that any one method of cancer treatment will be effective, such as with large or aggressive tumors, radiation therapy can be combined with surgery or chemotherapy. In some situations, a combination of all three types of treatment may be recommended.

How long does the entire treatment last, and what is the treatment schedule?

Radiation therapy is given in a series of treatments over many weeks. Depending on the type of cancer and the intent of treatment, it can range anywhere from 1 week to 2 months. This schedule helps to protect normal tissue by spreading out the total dose of radiation. The treatment area is designed to include all of the cancer and as little normal tissue as possible. The total dose used and the number of treatments depends on many factors including, the size and location of the cancer (i.e. which normal tissues will be within the treatment area), the general health of your pet, and the type of cancer present. Your veterinary radiation oncologist will review the specifics of your pet’s treatment plan with your before and during treatment.

What are the side effects of treatment?

Most side effects that occur during radiation therapy, although unpleasant, are usually not serious, and are almost always limited to the area being treated. Many animals develop skin changes in the area being treated. A redness of the skin may develop near the end of, or after, radiation therapy. This may progress to a dry or moist skin reaction, which resembles a severe sunburn or blistering rash. This "radiation dermatitis" may cause your pet to rub or scratch, but it is important you try to keep your pet from doing this. Your veterinarian may prescribe medication and/or physical means to prevent rubbing and scratching. 

Hair loss in the treated area is common and may persist for some time, but regrowth occurs in many patients. The color of the re-growing hair and skin in the treated area may change.

It is unusual for animals to become nauseated and have vomiting/diarrhea as a result of radiation therapy. This will usually only occur if large portions of the abdomen are irradiated. 

Pets usually experiences acute side effects for 3-4 weeks. Chronic side effects, when they occur, develop gradually over months to years. This includes fibrosis (scar tissue formation) and decreased ability to heal a wound or bone fracture. There is also a very small risk of a secondary cancer within the radiation site many years after treatment is administered.

These are the common side effects of radiation therapy, although the severity changes from patient to patient. Side effects involving other tissues that may be within the radiation treatment area (such as the eye, mucous membranes, and bone) will be discussed with you on an individual basis.

What happens after the treatment is over?

It is important for your veterinarian to examine your pet periodically after radiation therapy. This will allow normal tissue side effects to be monitored, and the effect of the radiation on the tumor to be evaluated. Specific follow-up schedules vary with each patient, but usually include a visit 2-4 weeks after the completion of radiation. 

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