The discovery and application of therapeutic radiation dates back to the late 1800s. Radiation is a term used to describe a variety of different kinds of energy. For example, we are exposed to ultraviolet radiation from the sun every day. Radiation therapists harness these different kinds of energies and direct them at tumors to kill tumor cells.
Radiation can be delivered from a distance (teletherapy) or very close to the tumor (brachytherapy). Machines that deliver radiation from a distance include orthovoltage and megavoltage machines. Orthovoltage energy comes from a machine similar to those that produce diagnostic radiographs (X-rays) for people and animals, but the energy is much higher. Because this energy is absorbed more by some normal tissues than others, there are side effects to tissues such as bone and skin when treating tumors. For this reason, this treatment is not as commonly used as megavoltage. Megavoltage radiation comes from cobalt and linear accelerator machines. The advantage of this kind of radiation is the ability to penetrate well into tissue, allowing us to treat all parts of a tumor.
The University of Missouri Veterinary Health Center uses a state-of-the-art linear accelerator for radiation therapy. The advantage of this over a cobalt machine is the ability to offer a wider variety of energies, which allows us to better tailor each radiation plan to each patient. This reduces side effects of radiation and improves our efficacy. Our machine is able to deliver both electrons and photons with conformational planning (described below). In addition, we are equipped to administer brachytherapy, including radioactive iodine for thyroid cancer, and implant therapy (implanting a radioactive bead).
At our facility, radiation planning is performed with a computer program that uses information gathered from a computerized tomogram (CT scan) of the tumor and the surrounding tissue. Radiation is most effective when there is tumor left behind that is too small to see with our eyes or CT scan but we can see where the tumor was. We use this information to tell the linear accelerator where we will direct our radiation therapy, and the computer tells us how much radiation each tissue will receive, including the tumor and each of the surrounding tissues. This is called conformational planning and allows us to minimize the dose of radiation to normal tissue while maximizing the dose of radiation to the tumor. We further reduce the amount of radiation to any given tissue by dividing each dose into multiple beams entering the patient from different directions, which means that tissue near each beam gets a percentage of the total dose but all beams converge on the tumor, giving it the full dose.
When radiation is administered, it must be divided into many small fractions. This allows normal tissue to recover between doses and minimizes the risk of delayed side effects. We calculate the total amount of radiation we would like to deliver and divide it into approximately 15 to 20 fractions. One fraction is given each day Monday through Friday. We do not administer radiation on Saturday and Sunday.
This process typically takes three to five weeks. Patients can go home at night if owners are able to transport them each day. Many patients travel from several hours away because radiation facilities for animals are sparse. These patients often stay in the hospital during the week and go home on the weekends, or even stay with us for the duration of the treatment.
We realize how difficult it is for pets and their families to be apart from each other, and we make every effort to ease this separation. Owners are contacted at least daily when their pets are in the hospital. Our patients may bring beds, toys and other comforts from home for their run; however, owners must be advised that these objects are sometimes misplaced and may be lost. In some cases we are able to arrange for foster care while the patient is here, allowing them to spend evenings at the home of a caregiver, typically a veterinary student or house officer with few or no pets.
Side effects are considered as early or late, with the short-term or early side effects occurring during and after the radiation treatment, and late effects occurring one to three years after treatment. Only tissue in the radiation field (the tumor and surrounding tissue) is affected.
Early side effects occur in all patients to some extent and heal in all patients but will depend on where the radiation is focused. If the tumor is deep beneath the skin, the dose of radiation to the skin will be low, but if the tumor or scar is at the surface, the skin will receive a higher dose of radiation and thus more side effects. These include skin changes that are similar to a sunburn. In dogs, skin becomes red, thick, moist and scabbed. In cats, the skin usually does not become moist but will turn red and scaly. Hair will fall out during and after treatment in most patients and will usually regrow over approximately six months. When hair regrows, it is often a different color and texture, usually white or gray. Other early effects include canker sore-like lesions in the mouth and decreased tear production.
Late effects, including degeneration of nerves and bone and scarring of normal tissues, occur in a small percentage of patients. These side effects are permanent, but can sometimes be treated surgically. In a small number of cases, radiation-induced cancer can occur years after treatment.
Radiation is commonly used for mast cell tumors, soft-tissue sarcomas, carcinomas, nasal tumors and to decrease pain associated with bone cancer. Radiation significantly improves the length and quality of life for most patients. Early side effects are tolerable for most patients, and we are proactive about controlling pain and inflammation.
Other forms of radiation therapy, such as radioactive iodine for thyroid cancer and samarium for bone tumors, are delivered as brachytherapy. These are radioactive substances that are targeted to a specific diseased tissue in the body. This minimizes radiation to the normal tissues and maximizes radiation to the tumor tissue. Samarium (brand name Quadramet) was developed at the University of Missouri and has been used to reduce the pain associated with primary bone cancer (osteosarcoma) or bone metastasis (spread of cancer to the bones) in dogs. In people it is used for breast and prostate cancer that has spread to the bones.
To consider a pet for radiation therapy of any kind, we ask that an appointment be made through the front desk (573-882-7821) for an oncology consultation. We fully evaluate each pet and all available treatment options to determine when and if radiation should be used as part of therapy.
