Radiation therapy is a common treatment modality for pet cancer. It uses ionizing radiation to damage the DNA in tumor cells, resulting in tumor cell death. Radiation therapy, like surgery, can offer local control of solid tumors such as carcinomas and sarcomas. Radiation therapy can also be used to provide relief of pain or improve function in patients with advanced cancers. Radiation therapy has been used for over 100 years, but technological advances in the last decade have vastly improved the ability to deliver dose specifically to the tumor while sparing normal tissue structures.

Types of radiation therapy

The most common type of radiation therapy is external beam radiation therapy, also known as teletherapy. Teletherapy is delivered by a radiation-producing machine like a linear accelerator, or from a machine housing a radioactive source, such as a cobalt machine. The patient is precisely positioned on a table, also called a couch, near the machine. Radiation travels from the machine to the patient, where the radiation “dose” is delivered to the tumor and surrounding normal tissues. While the patient remains in position, the machine revolves around the patient so that radiation is delivered from many different angles. Each treatment takes just a few minutes and does not cause any discomfort.

How does radiation therapy work?

Radiation therapy kills cells by damaging the DNA. The damage is from the localized release of ionizing radiation, which can damage the DNA directly, or more commonly, through the formation of oxygen free radicals. The cells generally do not die until the cell divides. One of the reasons it is effective against cancer cells is that cancer cells are routinely dividing. Radiation can also cause a more immediate death in some cancer cells, called apoptotic cell death.

Goals of radiation therapy

Typically, the aim of radiation therapy is long term tumor control called radiation therapy with curative intent. Depending on the tumor, most veterinary patients treated with curative intent protocols receive treatment over a 3-4-week period. A small “fraction” of radiation is delivered each day. Sometimes, depending on the location, there can be side effects, known as acute effects from this type of treatment. New technology is helping minimize acute effects.

Sometimes radiation is administered to relieve the patient of pain and compromising symptoms and/or improve quality of life. This is referred to as palliative radiation therapy. Palliative protocols are most commonly used when the patient has advanced cancer, metastasis, or some other critical condition that would limit life expectancy. These protocols vary and may involve weekly treatments or treatments given over the course of a few days. Palliative radiation therapy usually relieves pain and may even shrink the tumors a bit while rarely causing acute effects. Unfortunately, the duration of tumor control is far shorter than patients treated with more aggressive (curative) protocols.

Side effects of radiation therapy

Patients treated with curative intent, fractionated radiation therapy may develop side effects, called acute effects during or shortly after treatment. New radiation technological advances, such as the Varian Trilogy linear accelerator at CSU, can decrease the severity of, or even eliminate acute effects for tumors in some parts of the body. Nasal tumors, bladder and prostate cancers, oral tumors, and brain tumors now have minimal acute effects. Unfortunately, when tumors are located close to the skin surface, it is necessary for the skin to receive radiation doses that may cause discomfort. Your clinician will discuss potential side effects with you before treatment.

What is Stereotactic Radiation Therapy?

Stereotactic Radiation Therapy (SRT) delivers radiation to a tumor with the precision of a surgical scalpel. SRT protocols are generally delivered in 1-5 fractions (doses) on consecutive or alternating days. SRT requires special radiation therapy equipment including the ability to confirm the location of the patient and tumor right before therapy; that is the stereotactic aspect that the Trilogy provides. For some tumors, SRT can be used with curative intent. For other tumors it is used for palliation, but for far more durable palliation than what can be achieved using traditional methods.

SRT can be completed in a few days and acute radiation effects are minimal. However, it is not indicated for all tumor types/location. A radiation oncologist with knowledge about your pet’s specific tumor may be able to determine if this is an option.

What are the most common tumors treated with radiation therapy?

The most common veterinary tumors treated with radiation are brain tumors, pituitary tumors, tumors of the body and extremities (soft tissue sarcomas, mast cell tumors, vaccine-associated sarcomas), lymphoma, nasal tumors, oral tumors, bladder tumors, prostate tumors, perianal tumors, and bone tumors.

Flint Animal Cancer Center Radiation Oncology

The Flint Animal Cancer Center oncology service includes three radiation oncology faculty, four radiation oncology residents, and four radiation oncology technicians. They work with a team of medical and surgical oncologists and veterinary technicians to determine the best treatment plan for each pet cancer patient. Over the last four decades, Flint Animal Cancer Center radiation oncologists have pioneered several radiation protocols. They were the first veterinary oncology service in the U.S. to employ a linear accelerator for treatment of companion animals, the same technology used to treat people, and the first veterinary oncology service in the world to offer Stereotactic Radiation Therapy. With the help of clinical trials, today’s faculty continue to develop innovative treatments for pets with cancer.