Radiation Therapy

What is radiation therapy?

Radiation therapy is the medical use of high-dose radiation to destroy cancer cells. Radiation is a type of energy produced naturally by the sun, earth, and rocks and artificially by machines. Radiation is used for diagnostic as well as therapeutic purposes. Low-dose radiation, for example, is used to take radiographs (X-rays).

Cancer cells grow and divide faster than most normal cells. Radiation therapy works by damaging the cells’ DNA to interfere with cell replication and kill them. In this way, radiation can either shrink a tumor or destroy it.

When is radiation therapy used?


Depending on the type of tumor, radiation therapy may be used on its own or in combination with other treatments, such as surgery or chemotherapy. It is most effective with tumors that have rapidly dividing cells. Not all tumors are responsive to radiation.

Although several types of tumors can be treated with radiation therapy alone (e.g., nasal tumors, brain tumors, and certain types of lymphoma), radiation therapy is most commonly used to destroy or limit the growth of cancer cells left behind (i.e., microscopic disease) after a tumor has been surgically removed. Radiation therapy may also reduce the size of very large tumors before surgery, making the surgery more manageable.

How is radiation therapy administered?

Radiation therapy is typically administered while the pet is under a general anesthetic; the pet must be completely still during the procedure, as the radiation must be delivered with absolute precision. Depending on the treatment protocol, your pet may undergo several anesthetic procedures. The radiation delivery is usually quick, meaning your pet will only need to spend a brief time under anesthesia for each treatment.

Before commencing therapy, a CT scan is usually performed to ’mark’ the tumor's location. This allows the radiation to be delivered more precisely, minimizing damage to the surrounding tissues.

There are several types of radiation protocols in veterinary medicine:

  • Intensity-modulated radiation therapy (IMRT) is the traditional standard of care. This involves delivering radiation in small doses over several treatments (called fractions) to shrink or destroy tumors. With targeted therapy, the doses can be delivered directly to the tumor, avoiding damage to the surrounding healthy tissues and resulting in fewer side effects. Typically, the treatment schedule is from Monday to Friday for two to three weeks, although it can vary depending on the type of tumor and the individual pet. In the palliative care setting, where pain relief (i.e., comfort) rather than tumor shrinkage or destruction is the goal, IMRT may be given once or twice weekly as needed for several weeks.
  • Stereotactic radiation therapy (SRT) is a new therapy being adapted for veterinary medicine. Although SRT involves much higher doses of radiation over a few days, it is better able to exclude the surrounding normal tissues than conventional radiation therapy. Although side effects are still possible, the protocol is significantly shorter.
  • Strontium plesiotherapy is where high doses of radiation are delivered with a small probe to a very small and specific area and can be pursued for some types of tumors (i.e., nasal squamous cell carcinoma in cats).

What are the side effects of radiation therapy?

The side effects of radiation therapy can be split into two broad categories: early and late effects.

Early side effects are usually observed within two weeks after starting radiation therapy and can continue up until one month after the treatment has begun. These effects are usually inflammatory, with areas such as the skin and mucous membranes most commonly affected. Redness, irritation, and ulceration of the treated surface can develop (moist desquamation). Your veterinary oncologist will discuss the side effects that your pet may experience. Some possible effects are described below:

  • If the area being treated includes the skin and it is shaved, it may take longer for the hair to regrow, and it may regrow a different color. If dark-colored, it may be much lighter, and if light-colored, it may be much darker.
  • If the mouth is being treated, side effects can include excessive salivation, bad breath, the formation of plaques or ulcers, and secondary infections. This is called mucositis. Pets experiencing these side effects will be provided medications to treat the infections and pain.
  • If the eye is being treated, side effects such as blepharitis, conjunctivitis, and keratitis (inflammation of the eyelids, the surface of the eye, and the cornea, respectively) are possible. The eye can become dry due to a change in normal tear production and is susceptible to corneal ulceration (an open sore on the cornea). Cataracts and retinal damage are also possible and may lead to vision loss.
  • If the intestinal tract is within the radiation field, diarrhea may also be observed.

Late side effects usually occur more than six months after therapy. These effects also depend on the tumor and its surrounding tissues. Your veterinary oncologist will discuss the possible effects and what to watch for.