Beam direction and immobilization techniques

The use of modern localization techniques allows a radiotherapist to locate a tumor volume to a high degree of accuracy. The treatment of this volume by radiotherapy, while minimizing the radiation dose to the surrounding tissues, requires the use of accurately defined beams of radiation of suitable dimensions. Misalignment of any of these beams may mean that the radiotherapy treatment plan does not accurately describe the treatment. This may lead to parts of the tumor region being overdosed, and other parts underdosed, resulting in variations of tumor dose in excess of recommended tolerances. Sophisticated techniques have become available to enable radiation beams to be directed with the required accuracy, relative to marks on the surfaces of the patient. The accurate positioning of these marks is part of the planning procedure, using simulators or other diagnostic aids