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(Shimadzu, Kyoto, Japan) (Figure 4), a real tract radiation system9). It is combined with stereotactic radiation therapy to mainly treat lung or liver cancers. This technique, however, has drawbacks, Figure 4　(A) The tracking system on a medical linear accelerator (Linac). The white arrows indicate X-ray detecting boards and the asterisk indicates one of four X-ray sources placed under the floor. (B) We usually use two sets of X-ray systems to tract a gold fiducial marker placed near the target volume. While the marker is out of position, the Linac beam is off. If the marker moves into the predefined position on images from both directions, the radiation beam is on.Figure 3　Dose distributions in conventional conformal radiation therapy (A) and intensity modulation radiotherapy (B).including exposure to X-rays and the visualization of only the fiducial markers inserted near the target volume. A newly emerged MRI-guided treatment technique can detect the movement of 335