Ambient Dose Equivalent, Effective Dose and Mean Sv/Gy Conversion Coefficients from Scattered X-Ray Spectra in Mammography

Abstract

In this study, scattered X-ray distributions were produced by irradiating a tissue equivalent phantom under clinical mammography conditions by using Mo/Mo, Mo/Rh and W/Rh anode/filter combinations, for 25 and 30 kV tube voltages. Energy spectra of the scattered X-rays have been measured with a Cd0.9Zn0.1Te (CZT) detector for scattering angles between 30° and 165°. Experimental spectra have been corrected by a stripping procedure and then compared with calculated ones produced by semi-empirical models. In order to validate the adopted spectroscopic procedure, the values of HVL and air kerma calculated through the measured spectra were compared with those measured with an ionization chamber at the same experimental set-up at a non-clinical constant potential equipment with a tungsten anode and molybdenum filtration. Values of air kerma, ambient dose equivalent and effective dose have been evaluated through the X-ray spectra obtained in a clinical mammography unit. Mean conversion coefficients relating air kerma to ambient dose equivalent and effective dose from the scattered beams for all anode/filter combinations were evaluated. The mean conversion coefficient values from air kerma to ambient dose equivalent are between 0.21 and 0.54 Sv/Gy whereas the coefficients from air kerma to effective dose are between 0.036 and 0.11 Sv/Gy for the scattered radiation in the mammography energy range. Results show that for the scattered radiation beams the ambient dose equivalent gives an overestimate of the effective dose by a factor of about 5 in the mammography energy range. These results can be used in the control of the dose limits around a clinical unit and in the calculation of more realistic protective shielding barriers in mammography.