External beam radiation is the most widely used type of radiation therapy. The radiation is focused from machine outside the body onto the area affected by the cancer. It is much like getting an x-ray, but for a longer time. This type of radiation is most often given by machines called linear accelerators. The radiation is aimed at the tumor, but also affects the normal tissue it passes through on its way into and out of the body. External beam radiation allows large areas of the body to be treated and allows treatment of more than one area such as the main tumor and nearby lymph nodes. External radiation is usually given in daily treatments over several weeks. Several newer types of external beam radiation are making radiation therapy safer and more effective. These are described in the section. "What’s New in Radiation Therapy?" Treatment planning: The process of planning external beam radiation therapy is complex and may take several days to complete. It is, however, one of the most important parts of your radiation treatment. The doctor will design a treatment just for you. It will give the strongest dose of radiation to the cancer but will spare as much normal tissue as possible. This will help reduce the side effects that you experience. The first part of treatment planning is called simulation, sometimes referred to as a marking session. You will be asked to lie still on a table while the health care team works out the best treatment position for you. They will then mark the radiation field (also called treatment port), which is the exact place on your body where the beam of radiation will be aimed. Your doctor may use imaging tests such as a CT scan to check the size of the tumor, identify where it is more likely to spread, outline normal tissues that are in the treatment area, take measurements, and develop the treatment plan.