Validation of Monte Carlo simulation of mammography with TLD measurement and depth dose calculation with a detailed breast model

EPJ Web of Conferences Validation of Monte Carlo simulation of mammography with TLD measurement and depth dose calculation with a detailed breast model Wenjing Wang 1 2 Rui Qiu 1 2 Li Ren 1 2 Huan Liu 1 2 Zhen Wu 0 Chunyan Li 0 Junli Li 1 2 0 Nuctech Company Limited , Beijing , China 1 Key Laboratory of Particle & Radiation Imaging (Tsinghua University), Ministry of Education , Beijing , China 2 Department of Engineering Physics, Tsinghua University , Beijing , China Mean glandular dose (MGD) is not only determined by the compressed breast thickness (CBT) and the glandular content, but also by the distribution of glandular tissues in breast. Depth dose inside the breast in mammography has been widely concerned as glandular dose decreases rapidly with increasing depth. In this study, an experiment using thermo luminescent dosimeters (TLDs) was carried out to validate Monte Carlo simulations of mammography. Percent depth doses (PDDs) at different depth values were measured inside simple breast phantoms of different thicknesses. The experimental values were well consistent with the values calculated by Geant4. Then a detailed breast model with a CBT of 4 cm and a glandular content of 50%, which has been constructed in previous work, was used to study the effects of the distribution of glandular tissues in breast with Geant4. The breast model was reversed in direction of compression to get a reverse model with a different distribution of glandular tissues. Depth dose distributions and glandular tissue dose conversion coefficients were calculated. It revealed that the conversion coefficients were about 10% larger when the breast model was reversed, for glandular tissues in the reverse model are concentrated in the upper part of the model. 1 Introduction The average dose to the glandular tissue (mean glandular dose, MGD, or average glandular dose, AGD) is used to estimate the risk of breast cancer induced by ionizing radiation by ACR [ 1 ], EUREF [ 2 ] and IAEA [ 3 ]. MGD is estimated by the product of the measured incident air kerma at the upper surface of a breast and the conversion coefficient calculated using Monte Carlo codes. Conversion coefficients for breasts of different glandular contents and different compressed thicknesses (CBTs) are presented by Dance et al [ 4-7 ], which are adopted by the IAEA and EUREF protocols. Conversion coefficients presented by Wu et al [ 8, 9 ], which are adopted by the ACR protocol, are also listed by glandular content and CBT. The calculations are all based on a simple model of breast developed by Hammerstein et al [ 10 ]. However, MGD depends not only on the glandular content and the CBT, but also on the distribution of glandular tissues within the breast [ 11 ]. Depth dose distribution in breast in mammography, which is expressed as percent depth dose (PDD) or relative depth dose, has been widely concerned as the glandular dose decreases rapidly with increasing depth. Relative exposure was plotted as a function of depth in a phantom by Hammerstein et al [ 10 ], which was measured with Thermo Luminescent Dosimeters (TLDs) TLD-100. Comparison between depth exposure measured by Hammerstein et al [ 10 ] and that calculated with Monte Carlo codes was carried out by Wu et al [ 8 ]. A set of PDD curves were derived by a wide range of different combinations of target materials, filters and tube voltages with water phantom based on Monte Carlo simulation by Delis et al [ 12 ]. The PDD curves in 4.5 cm homogeneous breast phantom of 30% glandular and 70% adipose composition, 50% glandular and 50% adipose composition, and 70% glandular and 50% adipose composition were measured with TLD GR-200F by Tsai et al [ 13 ]. The PDD curves in 7 cm PMMA phantom for Mo/Mo, Mo/Rh and Rh/Rh target-filter combinations were measured with TLD-100 by Camargo-Mendoza et al [ 14 ]. Comparison of Monte carlo simulations and TLD-100H measurements of PDD in a 4 cm homogeneous breast phantom of 50% glandular and 50% adipose tissue by weight was carried out by Nigapruke et al [ 15 ] for verification of the EGSnrc MC code. The PPD curve was also measured with TLD-100 by Maria et al [ 16 ] in a phantom with composition of an average breast with 50% adipose and 50% glandular tissue, which was enveloped inside an adipose equivalent tissue case of 0.5 cm thickness. The TLD measurements were used to validate the dose values calculated using Monte Carlo programs of MCNPX and PENELOPE. In this study, TLD measurements with simple breast phantoms are used to validate the simulations with Geant4. And a detailed breast model constructed in previous work is used to study the effects of the distribution of glandular tissues in breast on MGD in mammography. values is shown in figure 4. The values obtained from simulations are well consistent with these obtained from measurements. 2 Materials and methods 2.1 Dose measurements with TLD The GE Senographe DS Mammomat was used to perfor (...truncated)