An apparatus (10) for irradiating a target includes a radiation source (12) for generating a radiation beam and a multiple leaf beam adjuster (16) for collimating and adjusting the shape of the radiation beam from the radiation source (12) that would be projected on the target. An image detector (17) detects generates an image signal of the target. In response to the image signal, a control module (18) generate a beam adjustment signal for controlling the beam adjuster (16), thereby enabling the radiation beam from the radiation source (12) to track the movement of the target.