STUDY OF BEAM ANGLES FOR DIFFERENT POSITIONS OF COLLIMATOR JAWS IN RADIOTHERAPY TREATMENT.

Abstract

Linear Accelerator Used for External-beam Radiation Therapy. Many types of external-beam radiation
therapy are delivered using a machine called a linear accelerator (also called a LINAC). A LINAC uses electricity to
form a stream of fast-moving subatomic particles. This creates high-energy radiation that may be used to treat cancer.
Collimators (beam limiting devices) are used in linear accelerators for radiotherapy treatments. They help to shape the
beam of radiation emerging from the machine and can limit the maximum field size of a beam.
The secondary collimators are situated after the primary collimator and consist of two pair of tungsten blocks (jaws),
upper jaws and lower jaws that restricts the radiation emerging from the head to determine square or rectangular field.
The secondary collimators are calibrated so that the collimator reading will give the corresponding field size at 100cm
FSD. Due to beam divergence the field size any other distance will be different and will be calculated by using similar
triangles method. For designing collimator jaws, the different beam angles were calculated for various field sizes.