11.10.5 - Determination Of Accuracy

Determination of mechanical accuracy

Light field concordance

The light field of the linac is designed to match the radiation field, including collimation by independent jaws and MLCs. To confirm accurate concordance, radiochromic field is placed within the field and the light field is turned on. The field borders are marked with pen. The linac is then used to deliver radiation to the field, and the concordance of the light field marks and the delivered treatment are checked. Accuracy should be within 2 mm.
The axis of the collimator and light field can also be checked by rotating the collimator through 180 degrees and marking the position of the light field at angles of 45 degrees.

Mechanical Isocentre

The mechanical isocentre involves checking the rotation of the collimator, gantry and couch.
The collimator isocentre can be checked by rotating the collimator through all directions while a 'center finder' or 'wriggler' is attached to the accessory tray. The center finder rests on graph paper with the assumed isocentre marked clearly. The displacement of the center finder as the collimator rotates allows determination of the actual isocentre position. The displacement should be under 2 mm.
The gantry rotation is checked with the same center finder with the addition of a horizontal rod. These are initially placed in close contact at the isocenter, and as the gantry rotates their position relative to each other is checked. Tolerance is < 1 mm.

Laser guides

The accuracy of the laser guides in determining isocentre position is of critical importance, as if patients are lined up incorrectly the radiation beam may miss the target. Tolerance of laser position is 2 mm. Laser position is checked by daily quality assurance tests.

Optical distance Indicators

The optical distance indicator displays a 'light ruler'. The ruler becomes focussed when the SSD equals the measurement on the ruler (ie. the 90 cm mark is focussed at an SSD of 90 cm). The optical distance indictor is checked by placing paper or a card at a physically measured distance from the treatment head and comparing this with the light ruler. Tolerance is within 2 mm.

Radiation Accuracy

Radiation Isocentre

The radiation isocentre is measured differently to the mechanical isocentre. The upper independent jaws are opened completely and the lower jaws are closed as far as possible, leaving just a thin slit for radiation to pass through. A piece of film is placed at the mechanical isocentre, and then exposures of the film are made at different collimator, gantry and couch angles. This creates a 'star' pattern, with the lines intersecting at the radiation isocentre. The lines should intersect within a 2 mm diameter circle around the mechanical isocentre.

Beam energy

For each energy of the beam, central axis depth dose curves should be generated using as small a dosimeter as possible. These depth dose curves should differ by less than 2% from published figures. This ensures that the desired beam properties are being obtained.

Field flatness

A beam should have a relatively uniform dose for at least 80% of its width. This can be determined by measuring dose across the beam in a water phantom. Dose should fluctuate by no less than 90% and no more than 103% of the central axis dose. Film is a useful way to measure beam flatness regularly as it is easily set up (compared with water phantoms), and absolute dosimetry is not required.


Wedges should be checked for the amount of angulation they are causing. Accuracy should be within 2 degrees of expected. Isodose distributions are the best way of checking wedge angulation, requiring water phantom measurements.