Many tumors of the dog and cat have been shown to be controllable, if not curable, by the use of radiation (fig.1)
Figure 1: A canine patient being positioned for treatment with cobalt -60 radiation.
. Ionizing radiation either kills tumor cells directly or may injure them enough so that they cannot divide again, making it impossible for the tumor to continue to grow. Radiation damages both tumor cells and normal cells in the area being irradiated, but the normal cells are (at least theoretically) able to repair themselves. Radiation injury will occur if the dose of radiation is too high for healing to occur, or if the radiation treatments are given too quickly for the normal cells to be able to heal before the next dose is given. Unfortunately, some radiation damage of normal tissue must be accepted if the cancer is to be killed.
Gamma rays may be emitted from certain elements (such as radium, cesium, and cobalt-60) as these materials decay, and the rays are used for treatment of cancer (fig.2).
Figure 2: The cobalt-60 radiation unit at Texas A&M University. 
Linear accelerators are devices which can produce high-energy, high-speed beams of charged particles, such as electrons; electron-beam therapy is most useful for treating very superficial lesions, such as those on the skin, or for deeper tumors which have radiation-sensitive structures in the field, since electrons can be targeted to penetrate deeply or superficially. Gamma radiation and electron-beam therapy are two kinds of external-beam radiotherapy (also known as teletherapy), so named because the area to be irradiated is treated over several minutes with a beam of radiation. After the radiation beam is turned off, no more radiation is present in the area. The patient is not radioactive when he or she is no longer under the radiation beam. External-beam radiotherapy is usually given in fractions, either daily or several times a week, until the total dose needed to kill the tumor has been reached. Fractionating the total dose allows the healthy tissue time to repair itself between treatments; tumor cells are theoretically not as able to heal themselves as are normal cells.
Another technique used to deliver radiation to tumors is brachytherapy, also known as implant therapy or internal radiotherapy (fig.3)
Figure 3: A cat with three implanted radioactive ribbons held in place by metal buttons. 
. In this procedure, radioactive implants (gold seeds, iridium beads, etc.) are placed directly into a tumor, organ, or site of tumor removal. Thus, the radiation dose is concentrated in a small area, and the patient is radioactive until the implants are removed or until they lose their radioactivity by decay. The animal will have to stay in the hospital for several days---usually about a week---in relative isolation, to prevent radiation exposure to humans or other animals. Intraoperative radiation therapy is a technique which is sometimes used to deliver a large dose of external-beam treatment to a tumor and its surrounding tissue during a surgical procedure. For some cancers, this is a particularly useful modality for delivering radiation therapy, since it spares surrounding radiosensitive tissues. A fourth form of irradiation is systemic radiotherapy, in which a radioactive isotope is given orally or by injection and "seeks out" a particular target tissue to irradiate (fig.4).
Figure 4: Radioactive iodine is injected to treat feline hyperthyroidism. 
This is a commonly used (and very effective) procedure to treat older cats with hyperthyroidism caused by a thyroid nodule; in these cats, radioactive iodine is given and taken up by the hyperactive thyroid gland(s). The cat is kept in isolation until most of the radioactivity has been eliminated from the body, and the pet is not of any danger to other pets or family members after it is sent home.
Although many tumors in small animals can be cured or controlled by radiation therapy, some tumors cannot be effectively treated using this modality. To be ideal for treatment with radiation, a tumor should be 1) of a radiosensitive cell type, 2) involve no very radiosensitive tissues that would have to be irradiated concurrently with the tumor, such as the gastrointestinal tract, 3) have borders that can be readily defined, and 4) be unlikely to spread to other organs (fig.5)
Figure 5: Hair loss outlines the radiation field in a dog with nasal carcinoma. 
. If the tumor cell type is not sensitive to radiation (bone and cartilage are particularly radioresistant), then it is unlikely that the radiation will produce any beneficial effect. If areas of microscopic tumor extend beyond the radiation field (the area that is to be irradiated), then the tumor will reoccur in these untreated areas. Finally, it is of little benefit if radiation therapy cures a tumor in the mouth, for example, but the animal is found to have metastasis to the lungs a few weeks later.
