Author: Huixiao Chen, Ethan Hartowicz, Lichung Ku, Tim Perez, Ray Yang 👨🔬
Affiliation: Salem Health, RefleXion Medical, Department of Therapeutic Radiology, Yale University School of Medicine 🌍
Purpose: To characterize MV-kV isocenter offset on a novel ring gantry radiotherapy system where the Linac, PET, kV-CT, and MV imaging subsystems rotate together at native operating speed of 60RPM.
Methods: A cube embedded with tungsten ball was kVCT-imaged at 60RPM, and localization shifts were applied to establish kV isocenter. While maintaining the gantry at 60RPM, the beam was collimated to 5x2cm and fired continuously while the megavoltage imager acquires every 3.6°. For baseline, a reference MV image set was acquired with static gantry at 45° increments. An analysis tool was developed to process each MV frame such that the averaged midpoint between inflection edges of the treatment field designates MV-isocenter, while the tungsten ball centroid represents kV-isocenter. MV-kV offsets in X and Y directions are plotted against gantry angle and compared to the reference static acquisition.
Results: For both static and 60RPM datasets, the MV-kV displacement was observed to oscillate as a function of gantry angle, exhibiting a simple sinusoid in X and higher order sum of sinusoids in Y. At 60RPM, the amplitude of deflections was dampened by nearly a factor of 2 compared to static. (0.92mm vs 1.84mm in X, 0.38mm vs 0.57mm in Y), indicating a stabilizing effect of high centripetal loading. The different characters of oscillatory patterns in X and Y suggest anisotropy in forces on the MLCs (X) vs the jaws (Y).
Conclusion: Performing the Winston-Lutz test at 60RPM captures dynamic forces on the collimation subsystem and gantry that better replicate the native operating conditions during treatment. The results reveal a nearly twofold reduction in MV-kV offset compared to static gantry MV acquisition, highlighting an important distinction of Winston Lutz testing under clinical operating conditions.