Beam collimators

Beam collimators are 'beam direction' devices used in the x-ray tube housing, along with an arrangement of mirrors and lights, in such a way that the light and x-ray fields match each other. They are made of lead shutters which completely absorb the photons, and thus reduce the patient dose as well as focus the radiation accordingly to the area of interest. They allow different projections of x-ray fields.

In linear accelerators (linac), the primary collimator may be constructed of depleted uranium as this material is approximately 1.6 times more dense than lead. The secondary motorised collimators which greater define the beam shape are constructed of lead or tungsten. Multileaf collimators (MLCs) which are now in widespread use in medical linacs consist of two collimator banks of thin tungsten 'leaves' with each bank consisting of 40 to 80 leaves (so a total of 80 to 160 leaves). This allows each collimator leaf to move independently under computer control. Multileaf collimators allow even more diverse field shapes to be created which can shield organs at risk whilst allowing the complex shape of the tumor bed to be irradiated to allow maximum cell kill.