Room: Exhibit Hall | Forum 7
Purpose: To produce an in-house quantitative tool for performing patient specific quality assurance on the Mevion S250i with Hyperscan pencil beam scanning proton therapy system.
Methods: MATLAB was used create a graphical user interface (GUI) that can read pertinent information from the machine log files and determine statistics for spot position and intensity modulation accuracy. The delivered spot positions obtained from the log file were compared against nominal positions as well as measurements performed on a Lynx PT scintillation detector. Spot position and MU accuracy per spot were also analyzed for a 5x5x5 cm3 uniform dose deposition centered at a depth of 10 cm in water. User defined failure thresholds for spot position and intensity modulation were offered to determine the overall quality of machine performance. In addition, the log file was converted to a patient specific input file for further Monte Carlo simulation using TOPAS with our in-house machine model.
Results: For an 11 by 11 uniform spot distribution spanning the maximum field size of 20x20 cm2, the mean distance between logfile-based determination of spot position and measurement was 0.85 mm with a standard deviation of 0.4 mm. An analysis of the log file for the cubic plan shows excellent agreement between nominal and actual spot positions with a threshold of 1.5 mm. In addition, a user defined failure threshold of 3% for spot intensity modulation shows a 94.3% passing rate.
Conclusion: With an accurate in-house quantitative tool for machine log file analysis, the clinical medical physicist can equip themselves with custom tools for routine machine performance evaluation as well as patient specific quality assurance. The combination of simulation results, actual spot positions, and actual spot intensity modulation gives the medical physicist the ability to separate daily machine performance from treatment planning system inaccuracy.