Absorbed dose per voxel, normalized by RPD (100% = 3.63E-14 Gy.photon−1). Coronal (a), sagittal (b) and transverse (c) slices of the VW model (RP represented as a small circle; PTV emphasized by the shaded region); and (d) the relative error per voxel in a transverse slice. PTV, planning target volume; RP, reference point; RPD, dose at the reference point.