Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Napoli, Napoli, Italy.
Università di Pisa, Dipartimento di Fisica, Pisa, Italy.
Phys Med Biol. 2021 Mar 12;66(6):065024. doi: 10.1088/1361-6560/abe2e9.
This work aims at calculating and releasing tabulated values of dose conversion coefficients, DgN, for mean glandular dose (MGD) estimates in digital breast tomosynthesis (DBT). The DgN coefficients are proposed as unique conversion coefficients for MGD estimates, in place of dose conversion coefficients in mammography (DgN or c, g, s triad as proposed in worldwide quality assurance protocols) used together with the T correction factor. DgN is the MGD per unit incident air kerma measured at the breast surface for a 0° projection and the entire tube load used for the scan. The dataset of polyenergetic DgN coefficients was derived via a Monte Carlo software based on the Geant4 toolkit. Dose coefficients were calculated for a grid of values of breast characteristics (breast thickness in the range 20-90 mm and glandular fraction by mass of 1%, 25%, 50%, 75%, 100%) and the simulated geometries, scan protocols, irradiation geometries and typical spectral qualities replicated those of six commercial DBT systems (GE SenoClaire, Hologic Selenia Dimensions, GE Senographe Pristina, Fujifilm Amulet Innovality, Siemens Mammomat Inspiration and IMS Giotto Class). For given breast characteristics, target/filter combination, tube voltage and half value layer (HVL), two spectra with two HVL values have been simulated in order to permit MGD estimates from experimental HVL values via mathematical interpolation from tabulated values. The adopted breast model assumes homogenous composition of glandular and adipose tissues; it includes a 1.45 mm thick skin envelope in place of the 4-5 mm envelope commonly adopted in dosimetry protocols. The simulation code was validated versus AAPM Task group 195 Monte Carlo reference data sets (absolute differences not higher than 1.1%) and by comparison to relative dosimetry measurements with radiochromic film in a PMMA test object (differences within the maximum experimental uncertainty of 11%). The calculated coefficients show maximum relative deviations of -17.6% and +6.1% from those provided by the DBT dose coefficients adopted in the EUREF protocol and of 1.5%, on average, from data in the AAPM TG223 report. A spreadsheet is provided for interpolating the tabulated DgN coefficients for arbitrary values of HVL, compressed breast thickness and glandular fraction, in the corresponding investigated ranges, for each DBT unit modeled in this work.
本工作旨在计算并发布用于数字乳腺断层摄影术（DBT）中平均腺体剂量（MGD）估计的剂量转换系数 DgN 的表格值。DgN 系数被提议作为 MGD 估计的独特转换系数，替代在全球质量保证协议中提出的乳腺摄影中的剂量转换系数（DgN 或 c、g、s 三联体）与 T 校正因子一起使用。DgN 是在 0°投影下测量的乳房表面每单位入射空气比释动能与用于扫描的整个管负载的 MGD。多能 DgN 系数数据集是通过基于 Geant4 工具包的蒙特卡罗软件推导得出的。对于乳房特性（乳房厚度范围为 20-90mm 和腺体分数质量为 1%、25%、50%、75%、100%）和模拟的几何形状、扫描协议、照射几何形状以及典型光谱质量的网格值，计算了剂量系数，这些特性和质量与六种商业 DBT 系统（GE SenoClaire、Hologic Selenia Dimensions、GE Senographe Pristina、Fujifilm Amulet Innovality、Siemens Mammomat Inspiration 和 IMS Giotto Class）相匹配。对于给定的乳房特性、靶/滤过组合、管电压和半价层（HVL），模拟了两种具有两种 HVL 值的光谱，以便通过从表格值进行数学内插来估计来自实验 HVL 值的 MGD。所采用的乳房模型假设腺体和脂肪组织的同质组成；它包括一个 1.45mm 厚的皮肤包膜，而不是剂量学协议中通常采用的 4-5mm 包膜。模拟代码经过 AAPM TG195 蒙特卡罗参考数据集的验证（绝对差异不超过 1.1%），并通过在 PMMA 测试对象中使用放射性色带进行相对剂量测量进行验证（差异在 11%的最大实验不确定度内）。计算出的系数与欧盟参考协议中采用的 DBT 剂量系数相比，最大相对偏差为-17.6%和+6.1%，与 AAPM TG223 报告中的数据相比，平均偏差为 1.5%。对于每个在本工作中建模的 DBT 单元，为任意 HVL、压缩乳房厚度和腺体分数值提供了用于内插表格 DgN 系数的电子表格，这些值在相应的研究范围内。
