Nonrigid registration of multitemporal CT and MR images for radiotherapy treatment planning

Abstract

External beam radiotherapy treats cancer lesions with ionizing radiation. Successful treatment requires a correct definition of the target volume. This is achieved using pre-treatment MR and CT images. However, due to changes in patient position, tumor size and organ location, adaptation of the treatment plan over the different treatment sessions might be wanted. This can be achieved with extra MR and CT images obtained during treatment. Bringing all images into a common reference frame, the initial segmentations can be propagated over time and the integrated dose can be correctly calculated. In this article, we show in two patients with rectum cancer and one with neck cancer that a significant change in tumor position and shape occurs. Our results show that nonrigid registration can correctly detect these shape and position changes in MR images. Validation was performed using manual delineations. For delineations of the manilible, parotid and submandibular gland in the head-and-neck patient, the maximal centroid error decreases from 6 mm to 2 mm, while the minimal Dice similarity criterium (DSC) overlap measure increases from 0.70 to 0.84. In the rectal cancer patients, the maximal centroid error drops from 15 mm to 5 mm, while the minimal DSC rises from 0.22 to 0.57. Similar experiments were performed on CT images. The validation here was infeasible due to significant inaccuracies in the manual delineations