Radiation qualities for calibration of dosimeters used in mammography

Abstract

For comprehensive quality control measurements in diagnostic radiography the number of semiconductor-based dosimeters is increasing. These compact dosimeters are, compared with ionization chambers, easy to handle and they provide several quantities with one exposure. Their inherent energy dependence of their response is more pronounced than for ionization chambers. Therefore, multiple compensation methods based on the radiation quality are developed by the manufacturers. To ensure that dosimeters are working correctly they should be calibrated regularly. The conditions in the calibration laboratory may vary significantly from the clinical ones. The range of clinically used radiation qualities is large and depends on the type of X ray machine. However, for calibration only a limited set of radiation qualities are available at the calibration laboratories. The international standard IEC 61267 [1] specifies the conditions for use in the determination of characteristics of diagnostic dosimeters. It includes mammographic X ray beam qualities only based on Mo Mo anode-filter combinations. Energy dependence of the response of eight semiconductor dosimeters and one ionization chamber were determined for clinically used radiation qualities with five different anode-filter combinations in tube voltage range from 25 kV to 35 kV. The maximum variations for six dosimeters were within the ±5% required by the IEC 61674 [2] standard. If a dosimeter is calibrated only with the standard radiation quality Mo Mo 28, the correction for the use with other radiation qualities can be up to 12%. The HVL and high voltage measurement functions were also tested, if available. For each beam quality the maximum variation was within 11% and 10%, respectively. Normally, the internal compensation of the energy dependence of response requires preselection of the radiation quality in the dosimetry software. If the radiation quality differs from the selected one, the maximum variation remains in the 5% range only in case of three dosimeters. To simulate the clinical radiation condition with compression paddle in the beam radiation qualities with additional filtration of PMMA were established. Even if the nominal radiation quality (anode-filter combination and tube voltage) is the same in calibration and clinical measurement, the additional filtration changes the spectra so that the response can increase up to 16%. This extended test demonstrates the radiation quality dependence of the response of semiconductor dosimeters. As well as provides some details about the built-in energy compensation applied. Based on this study errors and uncertainties related to different measurement and calibration scenarios can be estimated.