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

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 (DgN_p) 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). DgN_p coefficients were compared with values estimated using Boones' method and data from breast images (DICOM Organ Dose and VolparaDose calculations). The DgN_p were also evaluated in layers into the phantoms (depth-DgN_p) using both x-ray spectra and thermoluminescent dosimeters (TLD-100). Maximum differences between DgN_p 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 DgN_p 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 DgN_p estimated from the depth-DgN_p distributions differ up to 5.2% to the coefficients obtained using the pair incident-transmitted spectra to calculate the DgN_p directly in the whole phantom. The depth-DgN_p 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. (paper)