Determination of an optimal organ set to implement deformations to support four‐dimensional dose calculations in radiation therapy planning

Surface-based deformable image registration to generate a four-dimensional (4D) dose calculation in radiation treatment planning requires the selection of a set of organ contours to represent a basis set to generate anatomic deformation. The purpose of the present work was to determine the optimal set of organs needed to generate a basis set for deformation in treatment planning for thoracic tumors such that the required computations are minimized but dose accuracy is high. Using retrospectively reviewed records, we calculated 4D dose distributions based on treatment plans for 10 patients with thoracic tumors using a deformable model algorithm in a research version of a commercial radiation treatment planning system. Various combinations of organs (total lungs, heart, spinal cord, external body surface) were used to generate the basis set for deformations used in the calculations. The external surface contour did not have a noticeable effect on the dose calculation. Total lung, heart, and spinal cord together provided an adequate set of deformation organs to generate accurate dose deformations. The magnitude of calculated dose differences had no obvious relationship to tumor parameters, including site, histologic type, disease stage, extent of motion, or degree of centralization.