Author: Wesley S. Culberson, Albert Du, Ryan T. Flynn, Ryan Gardner, Alonso N. Gutierrez, Patrick M Hill, Daniel E. Hyer, Blake R. Smith, Nhan Vu, Karsten K. Wake 👨🔬
Affiliation: University of Wisconsin, Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison, Miami Cancer Institute, Baptist Health South Florida, Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, University of Iowa, Iowa Health Care 🌍
Purpose: In proton therapy, the substantial size of the gantry introduces sag, causing the proton isocenter to drift as a function of gantry angle. While the scanning magnets correct the beam’s isocenter alignment as a function of gantry angle, the nozzle frame remains unchanged. The Dynamic Collimation System (DCS), an accessory device capable of creating apertures via moving trimmer blades, must be mounted on the nozzle and requires similar corrections. This study introduces a look-up table to calibrate the software-defined DCS center point with the proton beam central axis (CAX) at each gantry angle.
Methods: The DCS trimmer positioning was calibrated at gantry 0° by setting the center point of the medial edges of the trimmers to proton beam CAX using the in-line kV imaging system on the IBA DN system at the Miami Cancer Institute. Beam’s-eye-view images of the DCS trimmers were captured in clockwise and counterclockwise rotations in 15° increments with the trimmers set to 1x1 cm2 field size defined at the trimmer plane. Trimmer positions relative to the proton beam CAX were determined at each angle and a look-up table was generated by averaging the deviations of the clockwise and counterclockwise arcs to correct DCS trimmer positions to align with proton beam CAX as a function of gantry angle.
Results: Before applying corrections, the largest deviations between the proton beam and DCS central axes were 1.20 mm and 0.45 mm in the x- and y-directions, respectively. After averaging the clockwise and counterclockwise corrections for all gantry angles, the corrected deviations were no greater than 0.23 mm in any direction.
Conclusion: Alignment of the DCS trimmers and proton beam CAX can be accurately corrected at all gantry angles using a measured look-up table. Residual deviations are minimal and are attributed to the averaging process to determine the corrections.