Shining a Light on Radiation Therapy for Pets with Cancer

Apr 3, 2018 / Advances in Medicine / Cancer Care / Cats / Dogs

[kim selting and hunter, radiation therapy]

Radiation makes cancer cells unable to reproduce

In the fight against cancer, doctors for people and pets have a range of weapons in their arsenal. Alone or in conjunction with surgery and chemotherapy, radiation therapy is useful for its ability to specifically target tumors. Dr. Kim Selting is a board certified radiation oncologist at the University of Illinois Veterinary Teaching Hospital in Urbana. She is one of the newest members of the cancer care team and is facilitating the acquisition of a linear accelerator in 2019 to advance radiation therapy services for pets.

[Dr. Kimbery Selting with Spike]

Dr. Selting with her own pup, Spike.

“The goals of radiation are to achieve the best possible tumor control and to decrease negative effects of the tumor, such as pain, while sparing normal tissues, optimizing both quality and quantity of life,” says Dr. Selting.

What Is Radiation?

Radiation can be broadly divided into two groups: electromagnetic and particulate radiation. Whether you know it or not, you interact with electromagnetic radiation every day. It consists of packets of energy, called photons, with no mass or charge. Common examples include microwaves, visible light, and radio waves.

The radiation used to treat cancer is different from these forms; it carries a much higher energy. The photons’ energy is high enough to ionize molecules, and thus is called ionizing radiation. Another form of ionizing radiation is called particulate radiation. It activates particles with mass, and it might or might not also carry a charge the way that electrons, neutrons, and protons do. Particulate radiation, with enough energy, can lead to similar effects in cells like ionization of molecules, but the particles interact differently with tissue than electromagnetic radiation does.

[hunter's radiograph and CT]

Hunter, a 12-year-old cat, received radiation therapy at the University of Illinois Veterinary Teaching Hospital. In the two CT images directly above, the top shows the large squamous cell carcinoma (tumor) on Hunter’s jaw as the white mass at the bottom left; the circle in the center is the breathing tube and the two round structures at left and right on top are his eyes. Below is an image from the side view; the black line bisecting the head is the breathing tube; beneath it is the tumorous jaw. The photo at the top of the page shows Dr. Selting positioning Hunter for the second of four scheduled radiation treatments. Hunter is under anesthesia and is kept warm with a red blanket.

“Radiation oncologists use a knowledge of how photons and particles interact with tissue to decide which is best to treat a given tumor,” says Dr. Selting.

How Does Radiation Therapy Work?

Radiation fights cancer by interacting with strands of DNA in cells, causing breakages that make the cell unable to replicate, meaning a cancer cell can no longer divide to make two daughter cells. When the damaged cell tries to divide, it collapses and is cleared away by the body.

Radiation is most often used to treat tumors that are locally aggressive and cannot be fully controlled with surgery. If the tumor is also aggressive throughout the body with a high chance of metastasis (spread of the cancer to other parts of the body), then radiation may be used for palliative care, to reduce the pain and other aspects of the tumor that affect the pet’s quality of life. Radiation may also be recommended as adjunct therapy, after surgical removal of the tumor, to target any cancer that could not be removed surgically.

Radiation therapy is performed several ways. Machines can shape the beam of radiation to the shape of the tumor and use precise positioning so that a very specific spot is treated each time.

“Unlike humans, our animal patients require anesthesia for their treatments to ensure we are able to get them in the precise position,” explains Dr. Selting.

How Much Radiation Is Given?

Doses of radiation therapy are calculated to optimize tumor control without harming nearby normal tissue. The total dose is divided into smaller doses, called fractions, that are given daily or every other day. The size of the fraction dose depends on the kind of tumor being treated as well as on how sensitive adjacent normal tissue is to radiation.

When the treatment goal is palliative, doses are lower to minimize side effects from the radiation. In more aggressive protocols, called definitive or curative protocols, higher total doses or higher dose intensity is used, which may cause short-term side effects.

What Are the Benefits and Side Effects of Radiation?

The most common side effects of radiation therapy in dogs are similar to those in people. The patients may experience skin redness or moisture for a short period of time. In cats the side effects are milder and may just be limited to short-term dry, flaky skin.

“Radiation can cause effects that are delayed by many months or even years, and though these are rare and don’t necessarily impact quality of life, they can be permanent,” says Dr. Selting.

How quickly radiation therapy works depends on the tumor. Some, especially those growing rapidly, may regress very quickly. Other may shrink some during treatment and continue to shrink over following weeks or months.

Many pet owners have concerns about the side effects of radiation treatment. If radiation therapy has been recommended for your pet, your veterinary oncologist will work with you to develop a plan that maximizes your pet’s quality of life while minimizing side effects.

If you have questions about radiation therapy or cancer treatment, contact your local veterinarian.

By Hannah Beers