Assessing the feasibility of using deformable registration for on-board multi-modality based target localization in radiation therapy

Purpose:

Cone beam computed tomography (CBCT) is typically used for on-board target localization in radiation therapy. However, CBCT has poor soft tissue contrast, which makes it extremely challenging to localize tumors in soft tissues such as liver, prostate and breast cancers. This study explores the feasibility of using deformable image registration (DIR) to generate on-board multi-modality images to improve the soft tissue contrast for target localization in radiation therapy.

Methods:

Patient CT or MR images are acquired during the simulation stage and are used as the prior images. CBCT images are acquired on-board for clinical target localization. B-spline based deformable registration is used to register MR or CT images with CBCT images to generate synthetic on-board MR/CT images, which are used for on-board target localization. Liver, prostate, and breast patient data were used in the study to investigate the feasibility of the method. The evaluation includes three aims: (1). Evaluate whether the registration and margin design in clinical practice is sufficient to ensure the coverage of the on-board tumor volume: the synthetic on-board MR/CT images are used to verify the target coverage based on the shifts determined by CT-CBCT registration in clinical practice; (2). Evaluate the potential for margin optimization based on the synthetic multi-modality imaging technique: shifts are determined by rigid registration between planning CT and synthetic on-board MR/CT, and the replacing PTV margin is determined to ensure coverage of the deformed tumor volume. (3). Evaluate the potential tolerant margin for DIR uncertainty based on the deformed tumor contour in planning CT images: shifts are determined by rigid registration between planning CT and the synthetic on-board MR/CT, and the tolerant margin is determined to cover the expanded deformed tumor volume in PTV.

Results:

In the process of DIR, using CT images as DIR prior images has better alignment than using MR as the prior images. The evaluation showed: (1). For the liver cases, the coverage of 6 in 8 cases is above 90%. For the breast cases, the coverage of 6 in 7 cases is above 90%. For the prostate cases, the coverage of all cases is above 94%. Most of the tumor volume defined by the on-board synthetic images were covered by the PTV based on the shifts applied in clinical practice. The 3 under-dosed cases are correlated with long,interfraction deviation treatment fraction, small volume, and zero-PTV margin design. (2). For 6 of the liver cases, 5 of the prostate cases, and all the breast cases, the synthetic images allowed the reduction of PTV margin, which is up to 6mm, 4mm, and 1.5 mm, respectively. For the cases with reduced optimized margin, the dose to the OAR and normal tissue can be spared based on the optimized margin while for the cases with increased optimized margin the increased dose is not significant. (3). For cases with reduced margin, the benefit margin for DIR uncertainty is available which are 2-4 mm, 1-5mm, and 2-3 mm for liver, prostate, and breast cases respectively.

Conclusion:

Our studies demonstrated the feasibilities of using on-board synthetic multi-modality imaging to improve the soft tissue contrast for target localization in low contrast regions. This new technique holds great promises to optimize the PTV margin and improve the treatment accuracy.