Author: David J. Carlson, Yiu-Hsin Chang, Huixiao Chen, Zhe (Jay) Chen, Blake Gaderlund, Ray Yang 👨🔬
Affiliation: RefleXion Medical, Department of Therapeutic Radiology, Yale University School of Medicine 🌍
Purpose: To address the root cause of dosimetric discrepancy for small central targets (<2cm) especially sensitive to geometric alignment of beamlets, by modeling higher-order oscillations of the collimation system on a RefleXion X1 gantry operating at 60 rotations-per-minute (60-RPM).
Methods: An in-house developed 4-ball phantom was imaged by an onboard megavoltage detector at 60 RPM to characterize gantry angle-specific beamlet offsets relative to the mechanical isocenter. Observed displacement of MLC leaves (X-direction) and jaws (Y-direction) were quantified and incorporated as gantry angle-specific leaf filters and jaw profiles in the beam model of the treatment planning system (TPS) for dose calculation. Ion chamber and film dose measurement of a 1.5cm central small target was compared to TPS calculations, with and without the applied corrections.
Results: MLC X-shift varied with gantry angle as a sinusoid of 0.2mm amplitude and 0.3mm offset. Jaw displacement followed an irregular trajectory totaling 0.4mm amplitude, with negative displacement peaking at 150°, positive displacement peaking at 315°, and a plateau near 220°. Incorporating gantry-angle-specific alignment offsets for collimation components improved dosimetric agreement (measurement vs. calculation) from 3.77% to 0.56% in point dose and from 84.2% to 93.6% in film QA passing rate (2%/2mm). Visually, a profile across the target evolved from a sharp monotonic peak to shallower bimodal peaks. Dose calculations for larger targets were negligibly affected. Implementation of the correction to the TPS added minimal computational complexity.
Conclusion: The in-house-developed 4-ball phantom provided an incisive diagnostic tool for characterizing MLC and jaw deflections under the high rotational forces of a 60RPM gantry. Beamlet alignment was identified as a key factor impacting small-target dosimetric congruence, addressed effectively through angle-specific modeling of collimation components. This approach demonstrates significant value for commissioning and quality assurance constancy.