Light field and crosshair quality assurance test using a simple lens system

We describe here a simple lens system to test the positioning of the field light source and mylar crosshair for radiation therapy linear accelerators. Ideally the light source for the field light and the crosshair should be centered on the axis of rotation of the collimator. The traditional method for testing this coincidence uses the shadow of the crosshair caused by the field light source. The shadow of the crosshair is dependent on the position of both the field light source and mylar crosshair. Geometrically it is possible for the field light source and the mylar crosshair to be off the axis of rotation of the collimator and still cause the shadow of the crosshair to be on the axis of rotation at some distance. Using a lens system the motion of the field light source and crosshair can be observed in sharp focus independently of one another as the collimator is rotated.