Isocentric technique

An isocentric technique is one where all beams used in a radiation treatment have a common focus point, a.k.a the isocentre.

The movements of the linac are threefold:
#the gantry rotates (like a big crane arm - red arrows)
#the collimator twists in the head of the gantry (blue arrows)
#the bed swings around in the floor (green arrows)

All of these movements in the modern linac occur around an axis that runs through the isocentre. In this way, if the centre of the target area in the patient's body is moved to coincide with the isocentre, then all motions of the machine will remain centred on the target.