An automatic deformable registration method to evaluate parotid glands shrinkage during radiotherapy treatment in tomotherapy

The aim of this study is to provide a method to track the anatomical alterations of parotid glands during radiation treatment. The method we implemented is based on an intensity-based free-form deformable registration to realign the planning kilo-voltage CT (kVCT) and the daily mega-voltage (MVCT) image sets and on an automatic contour propagation algorithm based on surface construction by triangular mesh. The accuracy of the method was evaluated through visual inspection and comparison with manual contours drawn by expert physicians. The uncertainties of automatic contours were similar to those of manual contours, in terms of parotid volumes and center-of-mass distances. The parotid volumes decreased with a median total loss of 21.7%. We observed an average medial shift of parotid center-of-mass of 3.1 mm from the external part toward the midline. The deformable registration method presented in this work provides an accurate tool for the automatic evaluation of parotid changes occurring during a radiotherapy treatment period.