Radiation therapy (RT) may be recommended as part of your pet’s treatment plan, depending on the type and location of the cancer. Typically, this is used when surgery is not possible or to follow surgery in an attempt to eliminate residual microscopic cancer cells. Some examples would be tumors in the nasal passage or brain, where surgery is not done or is more difficult, or soft tissue sarcomas that have been removed but the margins were not clear. Radiation can also be used in a palliative setting to shrink a tumor or slow the growth, or to alleviate pain, such as in bone cancer.
RT works by exposing tumor cells to particles that have energy, such as electrons or photons. These particles kill cancer cells by causing oxidative damage inside the cell, which harms DNA and other cellular organs. This damage prevents the cell from replicating. Depending on the rate of cell division (rate of growth) in the tumor, the full effects of radiation take several months to become apparent.
Before starting RT, a patient often receives a CT scan to map the tumor and plan treatment. The total dose of radiation is usually divided into small treatments called fractions. The number of fractions varies by the type of cancer and the type of radiation therapy protocol.
If treatment and a CT are pursued, your cat or dog will undergo general anesthesia to ensure that he or she remains still throughout the procedure. This step is important to ensure that the radiation is localized to the correct area each time. The procedure typically lasts only a few minutes. Prior to anesthesia, diagnostics are done to make sure your pet is a good candidate for anesthesia.
The most common protocols of radiation are stereotactic body radiation therapy (SBRT), intensity modulated radiation therapy (IMRT), definitive or full course RT, and palliative radiation therapy. There are also tumor-specific protocols, such as a hypofractionated protocol for melanoma.
Stereotactic body radiation therapy uses high doses of radiation with sub-millimeter accuracy. This type of radiation uses narrow margins to radiate a tumor to try to resolve the mass. Stereotactic radiation is typically given over one to three fractions within one week, and is usually preferred since there are less treatments and often fewer side effects.
Intensity-modulated and full course radiation therapy typically involve delivering smaller doses of radiation over a longer period, and it is typically given over 18-20 days, resulting in a higher dose of radiation exposure overall, as effects accumulate with each treatment.
Palliative radiation typically involves giving lower doses of radiation over a shorter period with the intent to reduce pain and discomfort. Palliative protocols are usually weekly or twice weekly treatments, although sometimes all within one week.
There are side effects that can result from radiation, which may vary by the location of the radiation therapy and the tissues involved. Most of the side effects can be managed with supportive medications and resolved quickly, having temporary or minimal impact on quality of life.
Your pet’s radiation oncologist and medical oncologist will help with the decisions of if radiation therapy would be beneficial to your pet and which protocol would work best.
