Author: Imad M. Ali, Nesreen Alsbou, Baraa Kalani 👨🔬
Affiliation: Oklahoma State University, University of Central Oklahoma, University of Oklahoma Health Sciences Center 🌍
Purpose: To develop a three-dimensional Winston-Lutz phantom to quantify the isocentricity of the gantry, couch, and on-board imager of the MEVION-S250i proton therapy machine.
Methods: A 3D-scintillation detector was utilized to measure the position and range of the proton spot-beams at various gantry and couch angles. A single proton spot-beam, with a maximum energy of approximately 227MeV, was delivered at different gantry angles (0°-180°) and couch angles (180°-360°). Variations in the position and range of the spot-beam were obtained from the maximum dose indicated shifts relative to the isocenter. A grid dose delivery pattern was used to examine position and size of spot beam position off-axis at distances ranging 10 cm from the isocenter.
Results: The proton spot beam size was approximately 3.5mm, providing superior spatial resolution compared to the star-shot films, which typically show a beam width of around 10mm. The isocentricity of the proton machine was quantified by measuring shifts in the spot-beam position at the isocenter across the full range of gantry and couch rotations. The shifts of the spot-beam isocenter ranged from 0.5-1.5mm as the gantry rotated from 0°-180°. The shifts in the couch’s isocenter position ranged from 0.65-2.5mm as the couch rotated (180°-360°). The proton spot-beam range was used to quantify the isocentricity along the beam path which varied by upto 1mm. The off-axis spot position varied between -1.2-1.6mm at different distances from the isocenter. The size of the spots varied between 2.5-3.9mm.
Conclusion: This Winston-Lutz phantom and detector system provided a more accurate assessment of the proton machine isocentricity in 3D compared to 2D-detectors and the traditional star-shot method. Additionally, this phantom offered integrated tools for measuring off-axis spot shifts and variations in spot size at different gantry and couch angles, making it a more accurate and comprehensive method for evaluating machine geometric performance.