Normalized glandular dose (DgN) coefficients from experimental mammographic x-ray spectra

Abstract

Mean glandular dose is the quantity used for dosimetry in mammography and depends on breast-related characteristics, such as thickness and density, and on the x-ray spectrum used for breast imaging. This work aims to present an experimentally-based method to derive polyenergetic normalized glandular dose coefficients (DgNp) from the spectral difference between x-ray spectra incident and transmitted through breast phantoms with glandular/adipose proportions of 30/70 and 50/50 and thicknesses up to 4.5 cm. The spectra were produced by a Mammomat 3000 Nova system using radiographic techniques commonly applied for imaging compressed breast thickness lower than 6 cm (Mo/Mo, Mo/Rh and W/Rh spectra at 26 and 28 kVp). DgNp coefficients were compared with values estimated using Boones’ method and data from breast images (DICOM Organ Dose and VolparaDose calculations). The DgNp were also evaluated in layers into the phantoms (depth-DgNp) using both x-ray spectra and thermoluminescent dosimeters (TLD-100). Maximum differences between DgNp from the method presented in this study and results using Boone’s method was 11%, with larger differences for Mo/Rh spectra in relation to the Mo/Mo. The DgNp maximum differences to the coefficients obtained using patient images were 8.0%, for the DgN calculated using Volpara and 6.4% for the DgN from DICOM Organ Dose, for a 4.5 cm breast phantom with 30% glandularity. The DgNp estimated from the depth-DgNp distributions differ up to 5.2% to the coefficients obtained using the pair incident-transmitted spectra to calculate the DgNp directly in the whole phantom. The depth-DgNp distributions estimated with TLDs were consistent with the results observed using the experimental spectra, with maximum difference of 3.9%. In conclusion, polyenergetic x-ray spectrometry proved to be an applicable tool for research in dosimetry in mammography allowing spectral characterization. This approach can also be useful for investigation of the influence of x-ray spectra on glandular dose.