Estimation of Organ Motion from 4D CT for 4D Radiation Therapy Planning of Lung Cancer

The goal of this paper is to automatically estimate the motion of the tumor and the internal organs from 4D CT and to extract the organ surfaces. Motion induced by breathing and heart beating is an important uncertainty in conformal external beam radiotherapy (RT) of lung tumors. 4D RT aims at compensating the geometry changes during irradiation by incorporating the motion into the treatment plan using 4D CT imagery. We establish two different methods to propagate organ models through the image time series, one based on deformable surface meshes, and the other based on volumetric B-spline registration. The methods are quantitatively evaluated on 8 3D CT images of the full breathing cycle of a patient with manually segmented lungs and heart. Both methods achieve good overall results, with mean errors of 1.02--1.33 mm and 0.78--2.05 mm for deformable surfaces and B-splines respectively. The deformable mesh is fast (40 seconds vs. 50 minutes), but accommodation of the heart and the tumor is currently not possible. B-spline registration estimates the motion of all structures in the image and their interior, but is susceptible to motion artifacts in CT.