Radiation Therapy for Pets
When your pet has been diagnosed with cancer, you will need to decide what treatment option(s) to pursue. While surgery and chemotherapy are mainstays of most cancer treatment protocols, radiation therapy (RT) is also available to treat localized disease. Radiation therapy is an effective way to treat many kinds of cancer, and can be used to treat tumors virtually anywhere in the body. Some cancers will respond well to RT when it is used as the only therapy, while others will have better outcomes when RT is combined with surgery and/or chemotherapy. Today’s blog will cover some important information about the use of RT for pets with cancer.
First off, what exactly is radiation therapy? Radiation therapy is the use of high energy rays or particles to treat a disease. These high energies damage the cell DNA and lead to cell death. While both the cancer cells and normal cells present in the treatment field are susceptible to damage, the normal cells have the capability to repair much of the damage that is caused, which allows these areas to heal and return to normal function after treatment is complete.
Radiation can be delivered by placing a radioactive material into or close to a tumor (known as brachytherapy), or it can delivered by a source located outside the body (known as teletherapy). Examples of the use of brachytherapy to treat diseases in pets include the use of radioactive iodine to treat cats with hyperthyroidism (high thyroid levels), and the use of samarium to treat pain in dogs with osteosarcoma (bone cancer). Teletherapy can be delivered by different machines. The current standard of care is the linear accelerator. This is basically a machine that can produce a high energy x-ray to cause cell death. The radiation beam that is produced travels through the body, and can damage both the tumor and normal tissue cells that are in its path. Many linear accelerators are also able to produce an electron beam. This is a preferred method of treatment for tumors of the skin and directly under the skin as the dosage of radiation decreases very quickly after reaching these superficial tissues. This allows the normal underlying tissues to receive a smaller radiation dose, thereby decreasing the side effects.
Linear Accelerator
Once you opt to pursue radiation therapy, a radiation oncologist will prescribe a total radiation dose for your pet’s specific tumor type, which is then given as multiple, smaller doses. Definitive therapy utilizes several treatments (often in the range of 16-18 or more) of a low dose of radiation to prevent tumor regrowth or shrink a tumor. Palliative therapy utilizes higher doses of radiation administered with fewer treatments (often in the range of 1-4) to control the clinical signs caused by the tumor. With both definitive and palliative treatments, advanced imaging with either a CT scan or MRI is required to allow for appropriate treatment planning.
Computer treatment plan of a patient with a tumor in front of the left ear
Many newer technologies have recently become available for veterinary radiation patients. These include stereotactic radiosurgery, also known as SRS, and intensity modulated radiation therapy, or IMRT. Both of these options have the capability to decrease the radiation dose to the normal tissues, thereby decreasing the unwanted radiation side effects, while increasing the dose that the tumor receives. Stereotactic radiosurgery also has the advantage of decreasing the total number of treatments required to definitively treat a tumor. While not yet widely available, any one of these may be the best option to treat certain cancers.
As I mentioned before, radiation patients will experience side effects in the normal tissue that is included in the treatment field. These side effects are classified as acute, which occur during and immediately after treatment, or chronic, which occur many months to years after treatment. The expected side effects will vary based on the area of the body that is treated.
The most common acute side effects occur in the skin, and are referred to as the radiation “burn.” The patient isn’t really burned, rather the normal outer skin cell layers are killed and sloughed off (known as desquamation), leaving the underlying tissue exposed. While this is unsightly and painful for the patient, these side effects often require minimal medical intervention, heal on their own, and do not cause significant long term problems.
Typical radiation desquamation lesion at the end of treatment
Chronic side effects do not always occur in the patient’s lifetime, but when they do, they can be debilitating. Examples of chronic side effects include fibrosis of the skin (scar tissue formation), the development of non-healing skin wounds, death of nervous system or bone tissue, or the development of a second tumor in the treatment field.
Unfortunately, RT for veterinary patients isn’t currently available in Oklahoma. This means that if RT is recommended for your pet, and you choose to pursue this option, you will need to travel outside of the state. There are many locations throughout the country that offer RT, and the three closest to us are: Kansas State University in Manhattan, KS; the University of Missouri in Columbia, MO; and hospitals in Dallas, TX.
Deciding to pursue RT for your pet is a big commitment in terms of time, money, and possibly travel. As with any treatment there are risks, but if RT has been recommended, the veterinarians caring for your pet believe that the benefits provided by the treatment outweigh the risks. We know that dealing with a cancer diagnosis in a loved one is hard, and sometimes making decisions concerning the best treatment option is even harder. I hope that this has provided you with some information to make the decision easier to make.
