Ghost Node Correction is a technique that is applied to some unstructured grid types, e.g., Nested or Quad-tree Grids, whose geometries do not inherently satisfy the requirements of MODFLOW-USG. Because Voronoi Grids by definition satisfy the constraints, ghost node correction is not required.
One of the geometric constraints for a Finite Volume grid is that the line formed by connecting the cell-center points of two adjacent Finite Volume cells must perpendicularly bisect the shared cell wall. While Voronoi Tesselations by definition fit this criteria, other grid types (such as Delaunary Triangulations and nested or Quadtree Grids) do not strictly conform to this constraint. In order to permit the mathematical solution of the Finite Volume Method for such grids, the USG package employs a technique called Ghost Node Correction (GNC) in which a temporary ‘ghost’ node is created within a cell such that it meets the geometric constraints and the equations may be solved. This solution at the ghost node is then interpolated (corrected) back to the actual cell center point, resulting in a Ghost Node Correction within each cell. (http://pubs.usgs.gov/tm/06/a45/pdf/tm6-A45.pdf)