Fig 13.

In this depicted example, the number of cycles experiencing the large gradient at the beam edge is on the order of 4 cycles and is proportional to the mean breathing displacement amplitude about the mean breathing frequency and inversely proportional to the couch velocity. If the beam had a square shape with no penumbra or scatter tail, then the only dose variation would be the variation in the 4 partially exposed voxels. However, the real beam has a gradient that spans much of its width. Therefore, a wide penumbra and scatter tail act to reduce the difference in roles of the beam width and the mean motion displacement amplitude. The conditions and parameters are the same as in Fig. 2.