Many radiotherapy treatment machines have built in Multi leaf collimators (MLC's). Multi leaf collimators are used to alter the shape of the beam. Without these, the linac can only treat square or rectangular shapes (treatment fields) without having to attach blocks of lead onto the machine, to shield out the radiation where it is not required.
MLC leaves range from 0.4mm - 1cm wide (depending on the linacs brand and type) and each leaf can be moved individually to match each patients specific radiotherapy treatment plan. This photo shows the head of the gantry - the irregular shape in the middle is created by the MLC's.
Reviewed: March 2015