A Monte Carlo approach to optimising a primary collimator design using EGSnrc

The primary collimator in a clinical linear accelerator (linac) plays a vital role in the radiation shielding of the machine. One objective of the collimation system is to reduce head leakage to less than 0.1 % relative to the dose on the central axis of the main beam. The objective of this study is to investigate the variation in different parameters of a primary collimator to identify a more optimal design through Monte Carlo simulations. Using the Monte-Carlo EGSnrc cavity application, water scoring regions up to 90°, away from the beam axis, were set up to investigate the properties of various tungsten primary collimator sizes and shapes with 6 MeV electrons incident to a target. For a fixed 12 cm length primary collimator, increasing the external radius from 6 cm to 12 cm, decreased side leakage measured at 90° by a factor of 10. Also, increasing the field size opening, increased leakage by a minimum of a factor of 5 at all angles. Extending the fixed 8 cm external radius primary collimator, from 8 cm to 14 cm length, made little difference to the leakage dose, for example, scoring at 45°. This reduction was a factor of 1.5. A conical frustum shape was found to be an optimal shape in terms of the level of shielding provided per unit volume of material compared to a cylinder.