SU-E-T-124: A Modified Winston Lutz Test Enabling Beam to Laser Angle Measurements

To present a modified Winston-Lutz-Test procedure able to measure beam and laser angles.

Room lasers have not only to indicate the isocenter spot but should also be aligned to the central beam axis. Therefore a modified WL test, based on a cube phantom made of low density foam material was developed. The classical steel sphere in the center is surrounded by 8 additional smaller spheres located near the cube corners. Surface markers on the cube indicate the position of the spheres and are used for easy setup to the lasers. Measurements are made with a field size covering all spheres in the well known way, ideally with a gantry mounted EPID. Result is an image of in total 9 spheres that is influenced by the distances and incoming beam directions. An automated template based detection algorithm then searches the image for the spheres as well as for the outside field boundaries. Knowing the phantom geometry, it is now easy to calculate the following parameters: Position of center sphere and laser to central axis of the beam, beam angle to the orientation of the phantom and the distance of the cube to the radiation source. Calculation result s then can be used to correct the phantom position and orientation. A transfer device equipped with a finder sight then allows to set the lasers.

Test measurements were taken at a Siemens Artiste. Here the detection accuracy for angles and positions was tested. For smaller angles the automated detection works quite well within an accuracy of around 0.1° (max error 0.2°). Position detection was below 1/10mm and showed clearly the effects of Gantry and collimator sag.

This method detects both, positions and angles of laser and beam, enabling a higher precision laser setup.