An isocentric technique is where all beams used in a radiation treatment

Radiation therapy

Radiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control...

 have a common focus point, a.k.a the isocentre.

Working

The movements of the linear particle accelerator

Linear particle accelerator

A linear particle accelerator is a type of particle accelerator that greatly increases the velocity of charged subatomic particles or ions by subjecting the charged particles to a series of oscillating electric potentials along a linear beamline; this method of particle acceleration was invented...

 (or linac) are threefold:

1. the gantry
   Gantry crane
   Gantry cranes, bridge cranes, and overhead cranes, are all types of cranes which lift objects by a hoist which is fitted in a hoist trolley and can move horizontally on a rail or pair of rails fitted under a beam...
   
    rotates (like a big crane arm)
2. the collimator
   Collimator
   A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction or to cause the spatial cross section of the beam to become smaller.- Optical collimators :In optics, a collimator may...
   
    twists in the head of the gantry
3. the bed swings around in the floor

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. This way, non-target areas will only receive short durations of radiation

Radiation

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

, reducing damage to them, while the target area receives constant radiation.