Dynamic Jaw Correction of Gravity-Induced Geometrical Effects on the Reflexion X1System for Improved Targeting Accuracy
Abstract
Purpose
Gravity exerts a force on the X1 causing gantry flexion, albeit small, that can be minimized to improve targeting accuracy while it is rotating. This study sought to demonstrate the mitigation of gravity-induced effects by improving the centering of the jaws using a custom-made phantom.
Methods
An in-house developed 4 ball phantom with 4 metallic balls on a plane and megavoltage detector (MVD) were used to characterize 1) jaw offset 2) centering of fields and alignment of the radiation isocenter (i.e. starshot). These data were modeled for jaw correction as function of gantry angle. For this test, the projected jaw MVD images and calculated jaw positions offsets at isocenter were used to keep the fields centered along Y axis at all gantry angles. This was done by adjusting the jaws in real-time and were verified using the MVD. Finally, film tests of jaw divergence per TG-148 were completed with and without jaw correction.
Results
The projected field motion ranges in IEC-Y improved from 0.48 mm before correction to 0.03 mm after correction, demonstrating the mitigation of jaw motion. Film jaw divergence testing showed mean jaw offset at isocenter (IEC Y) of -0.29 mm without correction, and 0.04 mm with correction.
Conclusion
The study showed that jaws can be used to correct gravity-induced structural deflections and was confirmed with external film measurements. This study improved field centering accuracy, which can potentially translate into improved targeting accuracy.