External beam radiation therapy procedure involves three phases:
- Treatment Planning
- Treatment Delivery
Simulation – Simulation helps determine the treatment position that helps maintain a specific position of the patient while receiving radiotherapy and obtain the necessary images for treatment planning. The patient is placed in the treatment position on a CT scanner.
The patient is held in a specific position during the simulation with the help of masks, pads or other immobilization devices. The patient should maintain the same position daily since this will be used for the treatment daily. The radiation therapist places small guiding markings (usually colored ink tattoos that fades eventually) on the patient’s body which guides the daily treatments. At simulation or during surgery, the marker seeds are placed in the targeted tumor or organ.
Treatment Planning – Three specialists, i.e., the dosimetrist, medical physicist and radiation oncologist are involved in treatment planning and they together determine the radiation dose required to be administered to the patient. The treatment plan is decided in a manner to deliver the appropriate dose to the tumor while minimizing dose to surrounding normal tissues.
Treatment Delivery – The treatment begins once the simulation and planning have been completed. The treatment procedure takes place in the treatment room where the patient is brought and placed on the treatment couch of the linear accelerator exactly as placed for simulation using the same immobilization devices. Positioning of the patient is done using the alignment lasers and the simulation marks. Imaging is used to verify the patient setup, some of the types of imaging used are x-rays, ultrasound and cone beam CT. The linear accelerator is turned on from outside the room and beams from different directions may be used for as long as several minutes for each field.