SU‐E‐T‐251: Developing a Daily Proton Beam Monitoring System

Abstract

Purpose:To develop a daily monitoring system for proton beam output check and beam uniformity check.Methods:Designed for continuously irradiated photon and electron beams with a field size of 20 cm × 20 cm, the daily output checker (Sun Nuclear, Inc.) is not suitable for monitoring proton beams with inter‐pulse beam‐off and a field size smaller than 14–16 cm in diameter. To allow such proton beam monitoring, the following tests were performed. 1. Absolute dose and array calibrations which accept continuous irradiation only, were performed using photon beams. 2. Five ion chambers within the central area of 8 cm × 8 cm were utilized to check constancy of output at the center of beam modulation and at distal edge and to check beam symmetry and flatness. 3. To simplify our evaluation, the array calibration was manually modified, such that all five chambers report equal values in spite of their differences in build‐up thicknesses. 4. The chamber at the lower‐right corner is placed under a buildup thickness that can offer dose measurement at the distal edge. This buildup thickness was determined by proton beam range measurements, which established buildup thickness for beam output measurement at the central chamber and range measurement at the corner chamber. 5. The beam‐off delay which allows receipt of pulsed irradiation was activated and optimal delay times were determined for each proton beam at 149.6, 185.6, and 249.5 MeV.Results:The above system was tested by miss‐steering proton beams and altering phantom thickness by 1 mm at a time. The system reliably monitored the beam with: 3% tolerance for beam flatness, symmetry and output. The range difference of 0.5 mm could be detected at all energies by setting a tolerance of 20%.Conclusion:A quick daily proton beam monitoring system was feasible.