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Development of An Image Registration Method Using Beam Path-Weighted Mutual Information for Patient Setup in Proton Beam Therapy

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S Takao

S Takao*, S Kawamura , H Ueda , T Matsuura , T Fujii , S Hirayama , S Shimizu , H Shirato , K Umegaki , Hokkaido University, Sapporo, Hokkaido


SU-I-GPD-J-12 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Proton beam therapy requires highly precise image guidance during patient setup to ensure accurate dose delivery. Cone-beam CT (CBCT) is expected to play an important role in reducing the uncertainties in patient setup. Mutual information (MI) is widely used as a measure of similarity between the planning CT and CBCT images to obtain the optimal translational and rotational shifts to be applied to the current patient position. We have developed an image registration method based on the concept of beam path-weighted mutual information (BWMI) and evaluated its efficacy for patient setup in proton beam therapy.

Methods: MI can be calculated from the joint probability distribution of the intensity values of the planning CT and CBCT images. By adding a weighting factor to each voxel in the geometric space, spatially-weighted marginal probability, and thus joint probability, can be obtained. During image registration with BWMI, higher weights were applied to the voxels within the beam path to improve the alignment of the target and tissues along this path. The efficacy for patient alignment was investigated using a planning CT image and a numerically deformed CBCT image of an anthropomorphic phantom. Residual errors in the position of the target and bony structure after image registration with BWMI were evaluated. The root mean square (RMS) error of the water equivalent path length (WEPL) from the body surface to the distal edge of the target was also examined.

Results: The image registration method reduced the RMS error of WEPL in patient alignment compared to bony anatomy matching and target matching methods. The RMS error was 0.6 mm in the phantom study.

Conclusion: A new image registration method based on the concept of BWMI has been developed and its efficacy for patient setup in proton beam therapy confirmed.

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