Continuous Isocentricity Test with Dynamic Axis Motions
Abstract
Purpose
To develop and validate a novel isocentricity test using continuous axis motion and imaging during beam delivery to characterize three-dimensional isocenter walkout.
Methods
Custom beam delivery sequences were created as custom XML-formatted machine instruction files to enable imaging during continuous axis motion of interest (e.g. couch, collimator, or gantry). Static reference images were subtracted from corresponding dynamic motion images, and geometric deviation from the motion was calculated and used to determine three-dimensional isocenter walkout for the gantry, collimator, and couch. For couch rotation, multiple gantry acquisition angles were combined to resolve the vertical component of couch-induced isocenter deviation.
Results
Subtracted image analysis produced clear and intuitive visualizations of isocenter walkout, with asymmetric intensity patterns and overlaid radial markers indicating walkout directionality. This approach enabled clear identification of collimator and in-plane couch walkout across the full range of motion, with walkout magnitude estimated as radial displacement using concentric reference circles at known spacing. Vertical couch walkout required an additional geometric calculation using paired oblique acquisitions in order to capture full couch motion without gantry collision. Dynamic gantry acquisitions and image subtraction revealed the overall magnitude of the gantry-induced isocenter walkout, but axis-specific directionality was not directly resolvable.
Conclusion
This work demonstrates the feasibility of a dynamic and continuous-motion isocenter testing approach for visualizing isocenter walkout during dynamic beam delivery. The method provides clear axis-specific information for collimator and couch motion, while gantry rotation imaging currently yields walkout magnitude without directional specificity. These results suggest that continuous imaging may offer a complementary and potentially more comprehensive alternative to the conventional static imaging-based Winston-Lutz measurements, particularly if gantry-specific dimensionality can be resolved.