Radiation Preparation

The following information has been extracted from various locations on the internet:

Treatment Planning
Before radiation treatments begin, you will go through a treatment planning process called "simulation". This is necessary to confirm that the treatment machine will treat the exact area requiring the radiation (called the treatment port). A map of the area on your body where the radiation is to be delivered, referred to as the treatment field(s), is created. A machine called a simulator, which is designed to mimic the movements and settings on the actual treatment machine (called a linear accelerator) that will be used to administer the radiation therapy to you, is used.
Throughout the process, X-ray films will be taken. The entire simulation takes one to two hours, but the actual time may vary slightly. Since you will be lying on a hard table under the simulator for most of this time, you may experience some pain or discomfort. Should this occur, tell the technician. Keep in mind that the time necessary to deliver the actual radiation therapy treatments is only minutes, not nearly as long as the time required for the simulation.
CT Simulation
In some situations, a special computerized tomographic (CT) scanner will be used along with simulation to help plan some radiation therapy treatments. In these instances, the CT scanning process will be performed in the Radiation Oncology Department several days prior to the simulation. Information from the CT scan is used to precisely locate the treatment fields. The CT scanner is specially designed to work with the other equipment in the Department, and is not a replacement for other diagnostic scans you may have received.
Tattoos
At the end of the simulation visit, it is necessary to put marks on you to outline the treatment field(s). These marks are in the form of very small tattoos, which are dots the size of a pinpoint or freckle, made using India ink. The process of tattooing causes some temporary discomfort. Generally, between 4 and 8 tattoo marks are placed, but sometimes more or less are necessary. In some treatment plans, a second set of tattoos may be necessary. While the tattoos are permanent, because of their very small size, they are not very noticeable. By using the tattoos, it will be possible for you to bathe or shower on a daily basis without worrying about removing these important marks that show where the radiation treatments are to be delivered. Also, if any additional radiation therapy is needed in the future, the tattoos serve as a permanent record of previous treatments. In very rare instances, tattoos will not be used.
Technical Planning Process
After simulation, details from the procedure are forwarded to medical radiation dosimetrists and medical physicists. These professionals perform highly technical calculations that will be used to set the treatment machine (linear accelerator).
Large alloy blocks may be placed inside the treatment machine to limit the amount of radiation delivered to your normal tissue in the treatment field. These blocks help decrease the development and/or severity of side effects caused by the treatments. Blocks are customized for each patient, in order to provide the best protection.
Set-Up
Several days after the simulation and after all calculations have been performed and custom blocks made, your treatment can begin.
Treatment machines (linear accelerators) resemble the simulator but are larger. You will be placed on the treatment table in the same position as you were on the simulator. Proper positioning usually takes 10 to 15 minutes. Once in place on the treatment table, a set of X-ray films will be taken. These films will be matched with the simulation films to ensure that the treatment is going to be delivered the same way as it was simulated. Occasionally, the match is not optimal. In these cases, adjustments will be made and will be checked by your attending physician. In rare cases, more adjustments are required and treatment may need to be postponed. Once the films and positioning are confirmed, a treatment will be delivered.
The actual administration of the radiation treatment usually takes only about 5 to 10 minutes. Factors that affect the total length of the treatment include the complexity of your treatment, the particular machine on which you are being treated and how quickly you can be positioned properly for treatment. On average, most patients spend a total of 20 to 30 minutes in the Department per day receiving their treatment.
On-Treatment Examinations
Once a week, repeat X-ray films will be taken to re-confirm proper positioning. These films will also be performed in most cases where there is a change in your treatment field or treatment plan. The X-rays taken during the course of radiation treatment are not used to help measure your progress or response to the radiation. Rather, they are only used to ensure that the position and treatment arrangement is appropriate.
You will also be examined at least once a week by your attending physician and resident. The examination day is generally on Thursday, just before or after your treatment is administered. These examinations are important because they give your physician the opportunity to evaluate your physical condition, answer any questions that you may have, and plan and coordinate future treatments.