Radiation Therapy

Portland Veterinary Specialists


Radiation, at levels thousands of times the amount used to produce a chest x-ray, can be used medically to kill cells.  Both normal cells and cancer cells are affected, but radiation treatment is designed to maximize tumor effect and minimize normal tissue damage.  This is one reason that radiation treatments are given as a series of small doses rather than one large dose.
Radiation therapy is used to treat localized disease either in a palliative or definitive protocol.  It can be used in the management of cancers that cannot be treated successfully by surgery or chemotherapy alone.  Typically, it is employed following surgery when there are tumor cells remaining after excision, either because of the nature of tumor growth or because complete surgical removal is not possible.  Radiation therapy can offer, in some instances, permanent control of a tumor.

Radiation Therapy Risks
    There are some risks involved with any type of cancer treatment.  In addition to cancer cells, some normal cells will also be damaged by the radiation potentially resulting in side effects. Usually these side effects are outweighed by the benefits of killing the cancer cells. In addition, radiation therapy requires the animal to be perfectly still during treatment.  Thus, general anesthesia is necessary for each treatment and this carries some small but potential risk.  The anesthetic agents used at NEVOG are very safe and short acting since the radiation treatment typically lasts only a few minutes and does not cause any pain or discomfort.  Risk under general anesthesia is higher if the animal is experiencing disease related the lungs, heart, liver or kidneys weather caused by cancer or some other disease process.  

How is the therapy given?
    In veterinary radiation oncology, a machine directs a radiation beam toward the cancer and some normal tissues around it.  At NEVOG, a linear accelerator with electron capabilities is used.  Radiation therapy is given in a series of treatments that encompass several weeks.  A definitive protocol consists of 15-20 treatments (called fractions) given daily Monday-Friday.  This schedule helps reduce long-term radiation side effects in normal tissues by giving smaller daily doses over a period of time.  The treatment area is calculated 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, the general health of your pet, and the type of cancer present.  The most important consideration is the total amount of radiation that can be administered to a patient without compromising the ability of healthy tissue to heal.  The treatment area may be marked with ink to enable the treatment to be directed to the same area every time.  Please maintain the marks as directed.  Sometimes radiographs or a CT scan of the treatment area are necessary to develop the appropriate radiation plan and for some patients, we use specific stabilization devices (ie. mask or mold of the head for brain tumors).
Even when long-term control is not possible, radiation therapy may improve a patient’s quality of life.  This is called palliative treatment.  This protocol consists of 3-5 large fractions of radiation given on a weekly, daily or every other day protocol depending on the cancer.  The side effects with this protocol are mild as it is designed to improve the patient’s comfort and quality of life while minimizing the negative impact.

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, 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?
At NEVOG, most patients receive treatments on a Monday-Friday schedule for a total of 15-20 treatments over the course of 3-4 weeks for definitive treatment or 3-5 weekly, daily or every other day treatments for palliative treatment.  Treatment does not occur on weekends, this is a rest period.  Patients may board in the hospital for the entire treatment period, go home on weekends, be dropped-off daily or treated as an outpatient (1 hour appointment).  Because of the anesthesia required, outpatients should not eat anything before a treatment from midnight the night before. Water may be provided throughout the night.  They can eat about 1-2 hours after the treatment.  Pets may be sleepy for several hours following each treatment.  You may also notice that your pet appears more tired towards the end of the treatment course.

What are the side effects of treatment?
    During treatment the oncologist will monitor the effect of the radiation on the cancer as well as on normal tissue.  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.  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.  It is important to keep your pet from causing self-trauma, as this will delay the healing phase.  In many cases we use an E. Collar or bandage to prevent self-trauma.  Ointments should be avoided during the moist (raw) phase as this may also delay healing.  You can clean the area with a warm compress and a mild soap.  Medications may also be prescribed to help manage the side effects.  Hairloss in the treated area is common and may persist for some time after the skin effects resolve.  Regrowth generally occurs in most patients and the color of the hair is usually white or gray.  The skin in the treatment field may also become darkly pigmented with time.
    It is unusual for animals to become nauseated and have vomiting or diarrhea as a result of radiation therapy.  This will usually only occur if portions of the abdomen are irradiated.  Some patients may experience diarrhea as a result of stress.  This usually resolves with diet or medical management.  Side effects involving other tissues that may be within the radiation treatment area (such as the eye, skin in the mouth, salivary glands of the oral cavity and bone) will be discussed with you on an individual basis.  The time from first appearance of acute side effects (those that happen in the immediate treatment period) until their resolution is usually 2-3 weeks.  Other side effects, if they occur, develop gradually over months to years and will be discussed as needed on an individual basis.

What happens after the treatment is over?
    It is important for your veterinarian or oncologist 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.  It is the goal of definitive radiation therapy to completely eradicate the cancer.  In some pets this happens and no evidence of the tumor persists.  In other pets the tumor may never completely disappear, but growth is stopped and the tumor is essentially controlled.  The specific results to be expected depend on many factors.  Details on the likelihood of success will be provided to you on an individual basis by the radiation oncologist.