New Daily QA phantom for proton beam spot scanning

Abstract

Purpose: The purpose of this research is to create a new design for a daily quality assurance phantom in proton pencil beam system. Using a single spot of a proton beam, this phantom should measure some of the factors necessary for daily QA dosimetry, such as Range Check, Beam Size, and Position Accuracy. Material and Method: Measured depth dose data is used to determine the depth 90% (d90%), which is an important point for accurate measurements of the QA phantom thicknesses. The QA phantom experimental tests were conducted using broad beam and pencil beam systems. To get the results of range confirmation and QA dosimetry measurements, both systems used a QA phantom with a scintillator system. In the broad beam system, the QA phantom design was a One-Step phantom, and for the pencil beam system, the QA phantom design was modified to a check pattern and named as a (Check phantom). The check phantom was tested in the broad beam system, with a small aperture collimator to simulate the pencil beam. Analytical calculations of depth dose were performed for each system condition to get the comparison results of experimental data and analytical calculation. The final results of experimental and analytical calculation were evaluated by using the B/A ratio method, where A is the Proximal dose of d90% and B is the Distal dose of d90%. Results: The results of the one-step phantom for experimental data and analytical calculation are summarized in the B/A ratio figure. Using the scintillator system with the QA phantom caused a quenching effect in the experimental data. The quenching was calculated analytically, and the results of the quenching calculation and experimental data show similarity in the B/A ratio data. The experimental data of the check phantom was analyzed by using the 2D Gaussian fitting method. To obtain the results of experimental data and analytical calculation in B/A ratio, the analytical calculation of depth dose for the check phantom is processed with the same calculating methods as in the one-step phantom, except incorporating the scattering effect of pencil beam dose distribution. The final results for both QA phantoms (One-Step and Check) show a consistent B/A ratio, and the new design check phantom confirmed its usability for daily QA phantom. Conclusion: The check phantom is eligible for daily QA phantom, with a one-spot of beam, it analyzes the required dosimetry measurements for daily QA.