Room: Exhibit Hall | Forum 7
Purpose: To present a four-year QA experience of the Mevion S250 compact proton therapy machine, a gantry-mounted double-scattered proton system in the Laurie Proton Therapy Center at Robert Wood Johnson University Hospital.
Methods: The daily, monthly and annual machine QA results and the clinical treatment field output calibrations (cGy/MU) were summarized. The daily dosimetric QA includes machine output, range, beam flatness/symmetry check. The monthly dosimetric QA includes machine output, range/modulation, and output dependence on gantry angle. The annual dosimetric QA includes machine output check for all 24 options, range/modulation and beam flatness/symmetry check. The monthly and annual mechanical QA includes the accuracy of gantry and couch isocentric rotation, snout and couch movement. For each treatment field, MU was determined by explicit output measurement. Meanwhile, a MU prediction model was established using the historical output measurements on this machine. Measured and model-predicted values were compared for 1080 treatment fields.
Results: For 94.3% and 98.9% of the output measurements, measured and model-predicted values agreed within Â±3% and Â±5% respectively, and the average difference was 0.1%Â±1.6%. Verification measurements were performed to verify the accuracy of the MU determined at the initial phase after commissioning and when there was large discrepancy between measured and model-predicted values. The average differences between annual QA and commissioning for absolute output, range, modulation, dose at modulation, flatness, symmetry, distal fall off distance were 0.1%Â±1.1%, 0.16Â±0.70mm, 0.32Â±1.61mm, -0.16Â±0.91%, 0.15Â±0.81%, 0.58Â±0.61%, 0.04Â±0.12mm, respectively. The radiation isocenter precision and radiation-imaging isocenter distance were both smaller than 1mm. The four-year daily dosimetric QA results, including output and range check for four options, are presented.
Conclusion: Overall, excellent agreement between routine QAs and commissioning results for Mevion S250 Proton Therapy System was observed in the data gathered over nearly 4 years of clinical use.