Radiation Therapy

What is radiation therapy?

Radiation therapy uses high doses of radiation to kill cancer cells. Radiation is a form of energy that occurs naturally in the sun, earth, and rocks. It can also be produced by machines.

In medicine, radiation is used both to diagnose and treat disease. For example, low doses of radiation are used to take radiographs (X-rays).

Cancer cells grow and divide more quickly than most normal cells. Radiation therapy damages the DNA inside these cells, making it harder for them to grow and multiply. As a result, tumors may shrink or be destroyed.

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 for tumors with rapidly dividing cells. Not all tumors respond 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 after a tumor has been surgically removed (microscopic disease). Radiation therapy may also reduce the size of very large tumors before surgery, making them easier to remove.

How is radiation therapy administered?

Radiation therapy is typically administered while the pet is under general anesthesia. The pet must be completely still during the procedure, as the radiation must be delivered with 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 starting therapy, a computed tomography (CT) scan is usually performed to identify the tumor's location. This allows the radiation to be delivered more precisely, minimizing damage to nearby healthy tissues. 

There are several types of radiation protocols in veterinary medicine:

Intensity-modulated radiation therapy (IMRT) delivers radiation in small doses over several treatments (called fractions) to shrink or destroy tumors. With this targeted therapy, the doses can be delivered directly to the tumor, reducing damage to the surrounding healthy tissues and resulting in fewer side effects. 

Typically, the treatment schedule is Monday through Friday for two to three weeks, although it can vary depending on the type of tumor and the individual pet. In a palliative care setting, pain relief may be the goal, rather than tumor shrinkage or destruction. In this case, IMRT may be given once or twice-weekly as needed for several weeks.

Stereotactic radiation therapy (SRT) involves much higher doses of radiation over a few days. SRT can more effectively spare the surrounding normal tissues than conventional radiation therapy. Although side effects are still possible, the treatment course is significantly shorter. 

Strontium plesiotherapy delivers high doses of radiation using a small probe to a very small and specific area and can be used for some types of tumors (e.g., 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 of starting radiation therapy and can continue for up to one month after the treatment has begun. These effects are usually caused by inflammation and most commonly affect the skin and mucous membranes. Redness, irritation, and ulceration of the treated surface can develop. Your veterinary oncologist will discuss the side effects that your pet may experience.

• If the area being treated includes shaved skin, it may take longer for the hair to regrow, and it may regrow a different color.
• If the mouth is being treated, one side effect may be mucositis, which involves excessive salivation, bad breath, plaques or ulcers, and secondary infections. Pets experiencing these side effects will be provided with medications to treat infection and pain.
• If the eye is being treated, side effects such as blepharitis, conjunctivitis, and keratitis (inflammation of the eyelids, eye surface, and cornea, respectively) are possible. The eye can become dry due to changes in normal tear production and may be more prone 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, your pet may develop diarrhea.

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