4D Treatment Optimization and Planning for Radiosurgery with Respiratory Motion Tracking

The CyberKnife® Robotic Radiosurgery System (Accuray Incorporated, Sunnyvale, CA) can treat targets that move with respiration using the Synchrony® Respiratory Motion Tracking System or the Xsight ℳ Lung Tracking System (Accuray Incorporated, Sunnyvale, CA). Alignment of each treatment beam with the moving target is maintained in real time by moving the beam dynamically with the target. The challenges of treatment planning for mobile targets are different for dynamic respiratory motion tracking than for conventional approaches such as motion-encompassing and respiratory gating methods that are common on gantry -based delivery de vices. Internal motion during respiration is not rigid, and thus positions of critical structures relative to the target and hence to the beam can change during respiration. The 4D Treatment Optimization and Planning feature, which recently became available in the MultiPlan® (Accuray Incorporated, Sunnyvale, CA) Treatment Planning System, is a new approach to four-dimensional (4D) treatment planning for motion tracking. It uses a 4D-CT image study to measure respiratory tissue motion and deformation and to account for the effect of motion and deformation on dose. The individual 3D-CT images are aligned so that the target coincides in each image. A tissue motion model is computed by performing non rigid registration of the individual 3D-CT images. Using the target -centric alignment and the deformation model, it is possible to calculate a dose distribution that takes into account both beam movement and soft tissue deformation. This dose distribution may be calculated before plan optimization and hence used to determine the desired beam geometry and weighting, or it may be calculated after plan optimization in order to review the effects of respiration on the dose isocontours and statistics for a given plan.

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