We have seen that DVHs and dose surfaces can agree, as long as both are based on voxels or points. But there is more to the story. Point based dose surfaces are created with essentially infinite resolution. That is, dose points are interpolated to the floating point accuracy of the machine, and those coordinates become the dose surface coordinates. With DVHs, dose points are interpolated to a finite grid. Although a 100x interpolation approached the dose-volume integral, it requires 100x in each dimension, or 100 cubed for 3D. This increases calculation time by a factor of 1 million! Even a 4x interpolation results in a 64x slowdown, turning 1 second into 1 minute, or .1 seconds into 6 seconds. As such, one is likely to use a much lower refinement for the DVH, and thus still see disagreement with the (relatively) infinite interpolation of the dose surfaces. |