Assessing the feasibility of using deformable registration for onboard multimodality-based target localization in radiation therapy

Aim: This study aims to explore the feasibility of using prior images and deformable image registration to generate onboard multimodality images to improve the soft tissue contrast of cone beam computed tomography (CBCT), for target localization in radiation therapy. Methods: B-spline-based deformable registration is used to register magnetic resonance/computed tomography (MR/CT) images with CBCT images to generate synthetic onboard MR/CT images for onboard target localization. Liver, prostate, and breast patient data were used in the study to investigate the feasibility of the method. Results: Most of the tumor volume defined by the onboard synthetic images was covered by the planning target volume (PTV) based on the shifts applied in clinical practice. For 6 liver cases, 5 prostate cases, and all the breast cases, the synthetic images allowed the reduction of the PTV margin to 1.5–7 mm, 1–4 mm, and 1–1.5 mm, respectively. The dose to the normal tissue can be reduced based on the optimized margin. Conclusion: This study demonstrated the feasibility of using onboard synthetic multimodality 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 in radiation therapy.