AlignRT uses an iterative closest point (ICP)‐based algorithm for surface matching. The algorithm registers the verification surface to the reference surface and calculates the shortest distance (by minimizing the mean squared error of these two surfaces). The distance is reported in the format of RTDs. We assume that the isocenter location relative to the surface does not change during the breath hold. Therefore, by knowing how much the surface deviates, we know where the isocenter has moved. The tangential beams can be moved accordingly and the delivered dose can, therefore, be calculated.