Patent Number: 
Section: abstract

A multi-leaf collimator that narrows a radiation field to a predetermined shape is provided with leaf blocks movable in the direction of the radiation field and having pattern images drawn along the direction of movement on a predetermined surface, and detection part acquiring an image of fixed-point via fixed-point observation in the direction of that predetermined surface and for detecting displacement of said leaf blocks based on the arranged locations of the pattern images existing in this image of fixed-point. Moreover, it is provided with detection part acquiring an image of fixed-point via fixed-point observation in the direction of that predetermined surface and for detecting the locations of the leaf blocks based on the arranged locations of the pattern images existing in this image of fixed-point. According to the present invention, displacement and locations of leaf blocks can be detected without making contact, and displacement due to the effect of backlash and gear wear or errors in detecting locations can be prevented. Therefore, regardless of backlash, the locations of the leaf blocks can be detected with high precision, and the radiation field can be matched to the shape of an affected part with high precision.