A method for quick alignment check of MLC leaves in tomotherapy Hi-Art II for QA

The alignment of MLC leaves with the beam defining jaws and the plane of rotation of the megavoltage X ray beam are done with film measurements and data from the xenon detectors of the Tomotherapy Hi-Art II machine during commissioning. The film analysis tools and the xenon detector data analysis tools are not readily available for the user. In order to verify alignment of MLC leaves, a simple and accurate method based on our previous experience with sequential tomotherapy using a MiMIC delivery system has been utilized. An EDR film is sandwiched between two 4 cm slabs of plastic water. The film is held vertically on the couch with its plane parallel to the horizontal green laser and laterally offset by 20 cm from the isocenter. Two procedures are created for static gantry positions of 90 and 270 deg. beam delivery with a sinogram which opens only the middle thirty two leaves for a 5 cm beam. The film is exposed in turn from the two static gantry angles and developed. The double exposed film shows a smaller rectangular field within a larger rectangular field. If the alignment is perfect, then the smaller rectangular field is symmetrically situated within the larger field. Also, all the sides of the smaller rectangle are parallel to the corresponding sides of the larger rectangle. Any twist between the MLC and the movable jaws can be observed from the lack of parallelism between the sides. A second EDR film sandwiched between two plastic water slabs is now placed similarly but this time at the isocenter with the horizontal green laser in the plane of the film. Two more procedures are created for static gantry angles of 90 and 270 deg. beam delivery with a sinogram which opens only the even numbered leaves for a 5 cm beam. The film is double exposed from 90 and 270 deg. with a 2 cm Y-direction displacement of the couch between the exposures. The alternate unexposed strips of the film from the 90 deg. exposure are filled by the exposure from the 270 deg. exposure, thus creating a uniform pattern of leaves separated by a thin gap between the adjacent leaves. If the alignment of the leaves is not perfect, then the leaf images will be overlapping. A visual check of the leaf image pattern will indicate whether or not the MLC is aligned properly. This simple check can be performed in the clinic at some regular frequency for quality assurance. Also, whenever a target or linac is replaced, this test can be performed for a quick alignment check. This check is also performed in a similar way for the other two cardinal gantry angles, i.e., 0 and 180 deg. In this case, the film sandwiched between the two slabs is kept horizontally on the couch. A quantitative analysis of the intensity pattern has also been done with a film analyser, but this method is a quick and simple visual check for alignment