A linear accelerator produces a beam of either electrons or very high energy X-rays. The radiation beam can be shaped and directed to match the tumor shape. The beam of radiation is directed on to the patient within an accuracy of 2mm. This is achieved by a combination of:
• control and shaping of the X-ray beam
• accurate patient positioning with respect to the X-ray beam.
Shaping of the beam can be achieved in three ways:
• use of the primary and secondary collimators. These are large tungsten blocks which can shape the X-ray beam into the appropriate square shape
• custom blocks -- these are cast blocks which are specifically for an individual patient to shield sensitive tissue or structures and can be made to the exact shape
• Multileaf Collimator (MLC) -- these are fine leaves (usually1cm wide) that are computer-controlled to match the shape of the tumor. Use of MLC is now common practice in many countries.