Positional accuracy of isodose lines as a function of dose matrix resolution

Abstract

The authors calculated the accuracy of isodose lines for a variety of clinical treatment techniques and beam energies using linear, quadratic and cubic spline interpolation methods. For a single beam, the greatest positional errors occur on either side of the steepest part of the penumbra (Niemierko and Goitein 1989). This is the region where the spatial second derivative of the dose is greatest. For plans with multiple beams, overall accuracy of isodose lines depends on the dose pattern as well as on the size of the geometric penumbra of the individual beams. Since computing the dose at a point is much faster using interpolation than physical simulation, the authors considered whether dose matrix calculation could be speeded by using higher order interpolation with fewer grid points for the same overall positional accuracy.