Figure 1

Cubic polynomial curve fitting basing on four markers. Cubic polynomial curve fitting from EPID (a) and DRR (b) images of the same phantom in the same position. Repeated cubic polynomial curve fittings from the EPID images of the same phantom in the same position based on two different groups four markers in EPID and DRR images (Eight different points in (c) for the EPID image and (e) for DRR image, red and blue curves in (d) for the EPID image and (e) for DRR image).