Transmission ionization chambers for measurements of air collision kerma integrated over beam area. Factors limiting the accuracy of calibration.

Kerma-area product meters (KAP meters) are frequently used in diagnostic radiology to measure the integral of air-collision kerma over an area A (integral of A Kc,air dA) perpendicular to the x-ray beam. In this work, a precise method for calibrating a KAP meter to measure integral of A Kc,air dA is described and calibration factors determined for a broad range of tube potentials (40-200 kV). The integral is determined using a large number of TL dosimeters spread over and outside the nominal field area defined as the area within 50% of maximum Kc,air. The method is compared to a simplified calibration method which approximates the integral by multiplying the kerma in the centre of the field by the nominal field area Anom. While the calibration factor using the precise method is independent of field area and distance from the source, that using the simplified method depends on both. This can be accounted for by field inhomogeneities caused by the heel effect, extrafocal radiation and scattered radiation from the KAP meter. The deviations between the calibration factors were as large as +/- 15% for collimator apertures of 5-100 cm2 and distances from the source of 50-160 cm. The uncertainty in the calibration factor using the precise method was carefully evaluated and the expanded relative uncertainty estimated to be +/- 3% with a confidence level of 95%.