INSTITUTE OF PHYSICS PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 51 (2006) 1077–1099 doi:10.1088/0031-9155/51/5/002 Automatic re-contouring in 4D radiotherapy * Weiguo Lu 1 , Gustavo H Olivera 1,2 , Quan Chen 1 , Ming-Li Chen 2 and Kenneth J Ruchala 1 1 TomoTherapy Inc., 1240 Deming Way, Madison, WI 53717, USA 2 University of Wisconsin-Madison, 1300 University Ave, Madison, WI 53705, USA E-mail: wlu@tomotherapy.com Received 22 July 2005, in final form 6 December 2005 Published 9 February 2006 Online at stacks.iop.org/PMB/51/1077 Abstract Delineating regions of interest (ROIs) on each phase of four-dimensional (4D) computed tomography (CT) images is an essential step for 4D radiotherapy. The requirement of manual phase-by-phase contouring prohibits the routine use of 4D radiotherapy. This paper develops an automatic re-contouring algorithm that combines techniques of deformable registration and surface construction. ROIs are manually contoured slice-by-slice in the reference phase image. A reference surface is constructed based on these reference contours using a triangulated surface construction technique. The deformable registration technique provides the voxel-to-voxel mapping between the reference phase and the test phase. The vertices of the reference surface are displaced in accordance with the deformation map, resulting in a deformed surface. The new contours are reconstructed by cutting the deformed surface slice-by-slice along the transversal, sagittal or coronal direction. Since both the inputs and outputs of our automatic re-contouring algorithm are contours, it is relatively easy to cope with any treatment planning system. We tested our automatic re-contouring algorithm using a deformable phantom and 4D CT images of six lung cancer patients. The proposed algorithm is validated by visual inspections and quantitative comparisons of the automatic re-contours with both the gold standard segmentations and the manual contours. Based on the automatic delineated ROIs, changes of tumour and sensitive structures during respiration are quantitatively analysed. This algorithm could also be used to re-contour daily images for treatment evaluation and adaptive radiotherapy. * This paper was in part presented at the AAPM meeting in Pittsburgh, Pennsylvania, USA, July 2004. 0031-9155/06/051077+23$30.00 © 2006 IOP Publishing Ltd Printed in the UK 1077