Here are some excerpts from a booklet by The City of Hope...
Radiation therapy (also called radiotherapy, x-ray therapy, cobalt or irradiation), is the use of high energy x-rays to kill cancer. Rarely it may be used to treat other diseases. In external radiation therapy, a treatment machine is used to beam the high energy x-rays to the cancer. The radiation beam is silent, invisible and painless, similar to a diagnostic x-ray.
WHY RADIATION THERAPY IS USED
Radiation therapy is used in a variety of ways to treat different types of cancers. It may be used alone or in combination with other treatments such as surgery or chemotherapy. Radiation may be used to shrink tumors before surgery, or to destroy remaining cancer cells after surgery. It can cure certain types of cancers, prolong life by limiting disease growth and spread, and reduce pain or other symptoms by decreasing tumor size. The basic purpose of radiation therapy is to destroy cancer cells in a specific area of the body.
HOW RADIATION THERAPY WORKS
During radiation therapy, cancer cells are damaged, prevented from reproducing and die out. Treatments are planned so that surrounding normal tissues are shielded from the radiation beam as much as possible. Cancer cells are more sensitive to radiation than most surrounding normal issues; however, normal tissues are also affected by radiation. Normal tissues which do receive some radiation are better able to repair themselves than cancer cells. Radiation side-effects can be produced by the action of radiation on surrounding normal tissues. Dividing the radiation dose into small amounts over a period of weeks allows normal tissues to recover in the intervals between treatments. In this way, radiation side effects are minimized while the slow daily destruction of cancer cells takes place.
Some types of cancer are treated by internal, as well as external radiation. Internal radiation therapy (also called Brachytherapy, interstitial or intracavitary therapy), involves placing small amounts of a radioactive material directly into the tumor area of the body for a short period of time. This is usually done under a general anesthetic and the patient stays in the hospital for a few days while the implanted radioactive material is in place. If internal radiation is indicated for your care, you will receive a complete explanation of this treatment from your doctor and nurse.
Radiation treatments may produce side effects in some people; others may notice them very little or not at all. The appearance of side effects depends on the person, the cancer, the type and amount of treatment, and, most of all on the body area treated. Side effects of radiation are confined to the part of the body being treated. Thus, you will not lose the hall on your head unless your head is being treated. If side effects appear, they tend to do so very gradually. They may be noticed after 2 or 3 weeks of treatment, remain throughout the treatment period and then gradually disappear by 2 or 3 weeks after treatment is completed. Your doctor will tell you which side effects you are likely to experience and how long they will last If side effects occur, you will be given specific instruction on what to do to minimize them. If you think you are having treatment side effects, be sure to let your doctor or nurse now about them. Many side effects can be reduced by the use of certain medications, diet or other measures.