A tandem calibration method for kerma-area product meters

Full text: The kerma-area product (KAP) of an X ray beam is the surface integral of air kerma over a plane perpendicular to the beam. The quantity is commonly used in diagnostic radiology to evaluate radiation exposure to patients. It can be measured, closely according to definition, using a KAP meter with a transmission ionization chamber. To achieve adequate accuracy in measurements, appropriate calibration of the KAP meter is needed. An uncertainty of lower than 7% (with a coverage factor k = 2) is pursued in air kerma measurements in diagnostic X ray beams. The same level of accuracy can be seen as a challenging goal also for KAP measurements. According to international standard IEC 60580 a combined standard uncertainty of 25% (k 2) should not be exceeded in KAP measurements. The aim of this study was to investigate whether better accuracy can be achieved by improving the calibration procedures for KAP meters. The KAP meter should indicate the kerma-area product of the X ray beam delivered through the chamber to the patient. It is also generally recommended that calibration of a KAP meter of an X ray equipment (field KAP meter) should be performed using the actual X ray stand and irradiation geometry, mainly because of the characteristics of extra focal and stray radiation specific to the X ray equipment. A portable KAP meter can also be calibrated in a calibration laboratory but the effects of extra focal and stray radiation should be tested separately with the X ray equipment in which the KAP meter is used. In calibrations on user's site the KAP reference value is usually determined by approximating the surface integral by the product of the X ray field area and the air kerma measured in the centre of the field. This beam area method works well in uniform, sharp-edged X ray fields. Non-uniformities and irregularities of the field may cause distorted dependence on field size to the determined calibration coefficients. Another important source of uncertainty is the measurement of field size. The accuracy of calibration can be improved by measuring the reference KAP value as an integral according the definition. Larsson, et al. have used TLDs to measure this surface integral. In this work a tandem calibration method was introduced in which the reference KAP value is measured using a KAP meter. In this approach the reference KAP chamber is placed in the X ray beam simultaneously with the field KAP chamber, using a distance of 30 cm between the chambers. The reference KAP meter has to be calibrated for the incident beam. In tandem method the measurement distance is not critical and field area measurements are not needed. As the reference KAP value is based on the measurement of the surface integral, the nonuniformities of the X ray field are inherently included. Because of remarkable dependence of the response on energy, the main uncertainties arise from the interpolation of the calibration coefficient of the reference KAP meter between X ray beam qualities. For tandem method, preliminary results indicate that uncertainty of 5% or lower could be achieved in the calibration of a field KAP meter. However, to achieve this level of accuracy, a comprehensive calibration of the reference KAP meter is required. Independent of calibration methods, the aim of 7% uncertainty in patient measurements is possible to achieve only in limited conditions of use and (or) by instruments of improved quality. This work has a contribution to the IAEA Coordinated Research Project for Testing of the Implementation of the IAEA Code of Practice on Dosimetry in X ray Diagnostic Radiology. (author)