4.1 - Isocentre Of The Linac

The isocentre is the point in space about which the gantry of the linear accelerator, the treatment head of the linear accelerator, and the couch rotate.

  • The mechanical isocentre is the point in space about which the linear accelerator and couch rotate
  • The radiation isocentre is the point where the radiation beams intersect if the gantry, collimator or couch is rotated.

These two points need not be the same although ideally they should be.
During normal operations, the radiation isocentre is defined by a set of laser guides that are positioned at the side of the bunker and on the linear accelerator itself. The junction of all these laser beams is the location of the isocentre.

Determination of the Isocentre

During quality assurance, both the mechanical and the radiation isocentre must be identified.

QA of Mechanical Isocentre

The mechanical isocentre is defined by attaching a distance rod to the treatment head of the accelerator. The distance rod, with an attached marker, should be located at the mechanical isocentre. Graph paper is placed at this point, and the collimator is rotated through 360 degrees. The marker should move no more than 2 mm in any direction. Once the collimator isocentre has been identified, a second metal rod is placed to intersect with the distance rod. The gantry is then rotated through all angles, and the distance between these two rods is calculated.

QA of Radiation Isocentre

The radiation isocentre is typically identified by exposing a film dosimeters to multiple exposures with slit-like fields. One set of jaws is closed as far as possible; the other set opened as far as possible. The film is covered with bolus (eg slab phantom) to ensure adequate buildup. If the film is placed in the path of the beam, then a single straight line should be produced. The collimator, gantry or couch is then rotated the the field re-exposed. Performing this test on multiple angles for each of the collimator, gantry or couch allows determination of the isocentre an the error in is positioning. Typically the radiation isocentre is kept within 1 mm movement in all planes.