Interpretation of alignment checks

The results of some of the previous tests can indicate errors in alignment of the machine. The possible errors are described below. The first step in setting the system is to ensure that the crosswires do not move when the collimator is rotated.

1. The light source is not on the rotation axis of the collimator. The light source rotates with the collimator and therefore it is possible to set the optical penumbra trimmers and the crosswire so that the crosswire position does not move and the collimators are symmetric at all collimator stages. The alignment of the crosswires can however cheat because the light source-isocentre line differs from the axis of the field so this only coincides with the axis of rotation at one SSD which is confirmed by rotating the collimator at an extended SSD.
2. The light source is not at the correct distance from the isocentre. In this case the field size at extended SSD is not in correct proportion to the field size at the isocentre and coincidence of the radiation and the light field will not be correct at the extended SSD.
3. The light source mounting is loose. If the light source moves during the gantry rotation the alignment of the light source will be different at different orientations of the gantry. This can be detected by checking the rotation of the collimator at 0^o and 180^o positions of the gantry.
4. The optical field trimmers protrude too much or too little into the beam. If the optical trimmers are incorrectly adjusted it will be manifested in the deffirence between the radiation and the light field size.
5. The optical field trimmers are asymmetric. The light field will be asymmetric and the asymmetric jaws will change their roles if the head is rotated through 180^o. If there is a doubt about this it is useful to adjust the light field trimmers so that they do not protrude into the beam and before adjusting the light field it should be checked that the correct radiation field size is set. If the optical field is symmetric in spite of the trimmers are asymmetric this indicates that the light source or the radiation source is incorrectly set up (see Item 7).
6. The radiation source is not on the central axis of the machine. The radiation source does not move with collimator rotation. If it is not on the axis of rotation of the collimator there will be an asymmetric radiation field and the asymmetry will be equal and opposite for opposing jaws and will not change as the collimator is rotated.
7. The electron beam is striking the target at an angle other than 90^o If the electron beam strikes the target at the wrong angle this can cause the asymmetry of the penumbra. The flatness scans also show a misalignment of the
minimum in the centre of the beam with the geometric centre.