Research On Beam Profile Measurement System Based On Pixel Ionization Chamber

To ensure the safety of proton therapy and to address the need for beam profile monitoring at the entrance of the nozzle,this paper conducts a study of beam profile’s 2D monitoring based on a pixel ionization chamber,taking into account the beamspot rotation and the volume effect of pixel ionization chamber.The algorithm improvement,performance evaluation,and dosimetric error analysis based on Monte Carlo simulation were carried out,and a graphical nozzle beam profile online diagnostic software was designed and implemented.The paper introduces correlation coefficients in the conventional 2D Gaussian model to increase the ability to monitor abnormal beam such as beamspot rotation and incorrect distribution,and uses a combination of maximum connected domain extraction and noise threshold determination in data preprocessing to identify fake signal regions,resulting in an improved surface Gaussian fitting algorithm.Through numerical simulation experiments,the detection process of the pixel ionization chamber is simulated,and the fitting errors of the beamspot’s parameters are evaluated under different signal-to-noise ratios.In order to correct for the systematic error introduced by the volume effect of the pixel ionization chamber,the paper proposes to use the fitted error curve of the measured beamspot size to rectify this systematic error,unlike the complex rectification method using additional detectors.Finally,the fitting error of the improved Gaussian fitting algorithm under the condition of 20 dB signal-to-noise ratio is obtained: the fitting error of the beamspot’s position is less than 0.22 mm,the relative fitting error of the beamspot’s size is less than 4%,and the fitting error of the roates angles is less than 6.1%.To verify the effect of the fitting error of the beamspot parameters on the treatment dose at a signal-to-noise ratio of 20 dB,the maximum fitting error of the beamspot parameters at this time is taken into account in prescried beamspot parameters to construct a fitted beamspot,which is used to simulate the 3D irradiation process of the proton beam and to compare the difference of the dose field formed by the superposition of the fitted and prescried beamspots.First,a Monte Carlo simulation model of the nozzle and the treatment dose evaluation area was developed with the nozzle structure,and an irradiation plan was designed to cover the dose evaluation area uniformly.Then,the differences of the dose fields to be evaluated with respect to the standard dose fields were evaluated using the gamma index method and the relative difference method,respectively.The evaluation results show that the two dose fields are in high agreement.Among them,the gamma evaluation results showed that the pass rate of the evaluated points on each scan layer reached 100% at the criteria of 2 mm/2% and 3mm/3%.Thus,the maximum fitting error at 20 dB can meet the dosimetric requirements.Based on the improved Gaussian model and the pixel ionization chamber at the entrance of the nozzle,the paper designs and implements a graphical beam diagnosis software to improve the efficiency of beam diagnosis.Aiming at the real-time monitoring of the beam at the entrance of the nozzle,a real-time monitoring scheme of beam profile with real-time controller is given,and the periodic data upload from the pixel ionization chamber channel data to the treatment control system is implemented in the engineering to ensure the treatment data can be archived uniformly.