The goal of this project is to exploit the facilities of the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UW-ADCL), with its National Institute of Standards and Technology (NIST) traceable x-ray beams and associated equipment to investigated the transfer of the calibration of ionization chambers to measurement of mammographic dose. The I-ADCL has been calibrating ionization chambers since 1981. The end results and technology will then be made available to the NIST and the Food and Drug Administration (FDA) Center for Devices and Radiological Health (CDRH), as well as the community. Reciprocal visits between the I and NIST will be made. The standards for calibration of ionization chambers used for mammography beams have been a tungsten target and aluminum filter, whereas mammography employs a molybdenum target and molybdenum filter. We have preliminary results that show that this spectral difference might cause up to a 4% error in dose measurement,so that the bench marks for dose measurement, so that the bench marks for dose measurement and half value layer used by CDRH and the community are not accurate, possibly resulting in inaccuracies of 10%. Since it is estimated that 40 million mammograms are performed per year in the United States, these results affect a large population. There are three major components of this project to accomplish its goal; however, the major component is that of the free-air chamber measurements. Each component is aimed at determining the difference in ionization chamber calibration between the tungsten/aluminum and the molybdenum / molybdenum beam combinations. In addition, the significance of he calibrations on the dose determination and other characteristics of mammography will then be determined. The three major components of this project are: 1. Measure air kerma with free-air chamber 2. Measure spectra and kVp of target/filter combinations 3. Develop and modify Monte Carlo codes for low energy x-ray beams. Each of these components will aid in the determination of the calibration factors and the dose.
| DeWerd, L A; Micka, J A; Laird, R W et al. (2002) The effect of spectra on calibration and measurement with mammographic ionization chambers. Med Phys 29:2649-54 |
| Coletti, J G; Pearson, D W; DeWerd, L A et al. (1997) Comparison of exposure standards in the mammography x-ray region. Med Phys 24:1263-7 |
