Abstract
Deformable image registration (DIR) is a critical technic in adaptive radiotherapy (ART) for propagating contours between planning computerized tomography (CT) images and treatment CT/cone-beam CT (CBCT) images to account for organ deformation for treatment re-planning. To validate the ability and accuracy of DIR algorithms in organ at risk (OAR) contour mapping, ten intensity-based DIR strategies, which were classified into four categories—optical flow-based, demons-based, level-set-based and spline-based—were tested on planning CT and fractional CBCT images acquired from twenty-one head & neck (H&N) cancer patients who underwent 6~7-week intensity-modulated radiation therapy (IMRT). Three similarity metrics, i.e., the Dice similarity coefficient (DSC), the percentage error (PE) and the Hausdorff distance (HD), were employed to measure the agreement between the propagated contours and the physician-delineated ground truths of four OARs, including the vertebra (VTB), the vertebral foramen (VF), the parotid gland (PG) and the submandibular gland (SMG). It was found that the evaluated DIRs in this work did not necessarily outperform rigid registration. DIR performed better for bony structures than soft-tissue organs, and the DIR performance tended to vary for different ROIs with different degrees of deformation as the treatment proceeded. Generally, the optical flow-based DIR performed best, while the demons-based DIR usually ranked last except for a modified demons-based DISC used for CT-CBCT DIR. These experimental results suggest that the choice of a specific DIR algorithm depends on the image modality, anatomic site, magnitude of deformation and application. Therefore, careful examinations and modifications are required before accepting the auto-propagated contours, especially for automatic re-planning ART systems.
Highlights
Intensity-modulated radiation therapy (IMRT) plays a critical role in the management of head and neck (H&N) cancer patients [1]
In a typical online adaptive radiotherapy (ART) process, a cone-beam computed tomography (CBCT) scan is usually performed to obtain three-dimensional patient information prior to treatment, and a deformable image registration (DIR) technique is utilized to establish the correspondence between voxels in the planning computed tomography (CT) and cone-beam CT (CBCT) scans for 1) propagating the contoured region of interest (ROI) from one image to another [10, 11] and 2) deforming the re-planned dose maps to integrate the accumulated dose administered to the patient [12, 13]
There are a variety of studies that have assessed the quality of computerized tomography (CT)-CT Deformable image registration (DIR) algorithms in head & neck (H&N) cancer patients; for instance, Castadot et al [14] compared twelve voxel-based DIR strategies in ROI propagation performance using expert physician-drawn contours as benchmarks
Summary
Intensity-modulated radiation therapy (IMRT) plays a critical role in the management of head and neck (H&N) cancer patients [1]. Though a number of CT-CBCT DIR methods for different clinical sites have been developed [11, 17,18,19,20,21,22,23], it is acknowledged that some DIR algorithms may be more suitable for specific anatomies and image modalities due to the mathematical basis of the algorithms [24] In this sense, it is beneficial to assess the clinical acceptability of the available DIR algorithms to reduce the time and resources required for contour review and correction in the CBCT-based ART process [15]
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