Performance characteristics and long-term calibration stability of a beam monitor for a proton scanning gantry

A monitor for a uniformly scanned beam was designed and constructed by the Indiana University Cyclotron Facility for use in a clinical proton gantry at the Midwest Proton Radiotherapy Institute. The beam monitor is a thin-walled, wide-aperture ionization chamber, which provides information about dose, beam size, symmetry, flatness, and position. Several characteristics of the monitor’s performance were studied, including linearity in dose rate, reproducibility, recombination correction, and dependence on both radiation field size and gantry angle. Additionally, stability of the detector output was analyzed using daily monitor calibrations performed over a period of 21 months.The beam monitor was found to meet design requirements for linearity (±1%), calibration stability (±2%), and stability of response as a function of gantry angle (±1%). Beam monitor calibration statistics also revealed a sine-like yearly trend with a ±2% maximum deviation from the average. These and other beam monitor test results are presented and discussed in the context of the detector design. Design changes aimed at further improving the detector’s performance characteristics are proposed.