Lines-based rotating beam profiling algorithm using pixel ionization chamber with dead pixels

Abstract

In a pencil beam scanning proton therapy system, critical beam parameters, including the spot size, spot center position, and tilt angle, are monitored for security by beam profile detectors such as two-dimensional (2D) pixel-segmented ionization chambers. A method, named lines-based beam profiling (LBBP), is proposed to identify the five geometric parameters μx,μy,σx,σy,θ of rotating elliptical beam profiles using several lines (rows and columns) in the 2D detector data. The method provides solid results even in the presence of dead pixels, which can be located using a wavelet analysis. Simulation experiments were used to define the number of lines used by the LBBP method and to characterize its performance as a function of the signal-to-noise (SNR). For SNR greater than 30, the root mean square errors (RMSEs) of the beam spot size, spot center position, and rotation angle obtained with the LBBP method are less than 0.16 mm, 0.05 mm, and 0.78 degrees, respectively. The LBBP method was applied to the data collected by a pixel chamber illuminated by an X-ray source, demonstrating the capability of identifying dead pixels and providing more accurate measurements of the beam spot size and center position than the standard Gaussian fitting.