The effects of dose calculation resolution on dose accuracy for radiation therapy treatments of the lung. Part II. A comparison of dose distributions from an explicit lung model to dose distributions derived from a CT representation.

Purpose: Due to limitations in computer memory and computation time, typical radiation therapy treatments are calculated with a voxel dimension on the order of several millimeters. The anatomy below this practical resolution is approximated as a homogeneous region uniform in atomic composition and density. The purpose of this article is to examine whether the exclusion of anatomic structure below the practical dose calculation resolution produces deviations in the resulting dose distributions.

Methods: EGSnrc calculated dose distributions from the BRANCH lung model of Part I are compared and contrasted to dose distributions from a CT representation of the same BRANCH model for three different phases of the respiration cycle.

Results: The exclusion of branching structures below a CT resolution of 1 x 1 x 2 mm3 resulted in a deviation in dose. The deviation in dose was as high as 14% but was localized around the branching structures. There was no significant variation in the dose deviation as a function of either field size or lung density.

Conclusions: The exclusion of explicit branching structures of the lung in a CT representation creates localized deviations in dose. To ensure accurate dose calculations, CT resolution must be increased