Technical Note: A method for improving the calibration reproducibility of an ionization chamber detector array

PURPOSE: This paper describes an extension to a wide field calibration method implemented on a commercial detector array in order to improve the reproducibility of the calibration procedure. METHODS: Following the standard array calibration procedure, two additional 10×10 cm exposures were acquired for each array axis with the detector array shifted by ±10 cm in the transverse or axial axes, or by ±10√2 cm in the positive or negative diagonal axes. These exposures were compared with a final baseline 10×10 cm exposure captured with the detector repositioned at the isocenter. The measurements were used to calculate a linear off-axis correction gradient which was then applied to the stored calibration factors. RESULTS: The mean coefficient of variation between five repeat calibrations was reduced from 4.17% to 0.48% and the maximum percentage error in individual calibration factors was reduced from 6.46% to 0.77%. CONCLUSIONS: The reproducibility of the calibration factors of an ionization chamber array was increased by capturing a baseline exposure and two further off-axis readings per calibration axis.