Carleton Laboratory for Radiotherapy Physics, Physics Dept., Carleton University, Ottawa, ON, K1S 5B6, Canada.
Med Phys. 2019 Mar;46(3):1426-1436. doi: 10.1002/mp.13372. Epub 2019 Feb 4.
To investigate energy deposition in glandular tissues of the breast on macro- and microscopic length scales in the context of mammography.
Multiscale mammography models of breasts are developed, which include segmented, voxelized macroscopic tissue structure as well as nine regions of interest (ROIs) embedded throughout the breast tissue containing explicitly-modelled cells. Using a 30 kVp Mo/Mo spectrum, Monte Carlo (MC) techniques are used to calculate dose to ∼mm voxels containing glandular and/or adipose tissues, as well as energy deposition on cellular length scales. ROIs consist of at least 1000 mammary epithelial cells and ∼200 adipocytes; specific energy (energy imparted per unit mass; stochastic analogue of the absorbed dose) is calculated within mammary epithelial cell nuclei.
Macroscopic dose distributions within segmented breast tissue demonstrate considerable variation in energy deposition depending on depth and tissue structure. Doses to voxels containing glandular tissue vary between ∼0.1 and ∼4 times the mean glandular dose (MGD, averaged over the entire breast). Considering microscopic length scales, mean specific energies for mammary epithelial cell nuclei are ∼30% higher than the corresponding glandular voxel dose. Additionally, due to the stochastic nature of radiation, there is considerable variation in energy deposition throughout a cell population within a ROI: for a typical glandular voxel dose of 4 mGy, the standard deviation of the specific energy for mammary epithelial cell nuclei is 85% relative to the mean. Thus, for a glandular voxel dose of 4 mGy at the centre of the breast, corresponding mammary epithelial cell nuclei will receive specific energies up to ∼9 mGy (considering the upper end of the 1σ standard deviation of the specific energy), while a ROI located 2 cm closer to the radiation source will receive specific energies up to ∼40 mGy. Energy deposition within mammary epithelial cell nuclei is sensitive to cell model details including cellular elemental compositions and nucleus size, underlining the importance of realistic cellular models.
There is considerable variation in energy deposition on both macro- and microscopic length scales for mammography, with glandular voxel doses and corresponding cell nuclei specific energies many times higher than the MGD in parts of the breast. These results should be considered for radiation-induced cancer risk evaluation in mammography which has traditionally focused on a single metric such as the MGD.
在乳腺 X 光摄影的背景下，研究乳腺组织的宏观和微观尺度上的能量沉积。
开发了多尺度乳腺 X 光摄影模型，其中包括分段、体素化的宏观组织结构以及整个乳腺组织中嵌入的九个感兴趣区域（ROI），这些区域包含明确建模的细胞。使用 30 kVp Mo/Mo 光谱，蒙特卡罗（MC）技术用于计算包含腺体和/或脂肪组织的mm 体素的剂量，以及细胞长度尺度上的能量沉积。ROI 至少包含 1000 个乳腺上皮细胞和200 个脂肪细胞；在乳腺上皮细胞核内计算比能（单位质量上传递的能量；吸收剂量的随机模拟）。
分段乳腺组织内的宏观剂量分布显示，能量沉积取决于深度和组织结构而有很大差异。包含腺体组织的体素剂量在0.1 到4 倍平均腺体剂量（整个乳房平均）之间变化。考虑微观长度尺度，乳腺上皮细胞核的平均比能比相应的腺体体素剂量高30%。此外，由于辐射的随机性，在 ROI 内的细胞群体中存在很大的能量沉积变化：对于典型的腺体体素剂量 4 mGy，乳腺上皮细胞核的比能标准偏差相对于平均值为 85%。因此，对于乳腺中心的 4 mGy 腺体体素剂量，相应的乳腺上皮细胞核将接收高达9 mGy 的比能（考虑比能 1σ 标准偏差的上限），而离辐射源更近 2 cm 的 ROI 将接收高达~40 mGy 的比能。乳腺上皮细胞核内的能量沉积对细胞模型细节敏感，包括细胞元素组成和细胞核大小，这突出了真实细胞模型的重要性。
在乳腺 X 光摄影中，无论是在宏观还是微观尺度上，能量沉积都有很大的变化，腺体体素剂量和相应的细胞核比能比乳房某些部位的平均腺体剂量高很多倍。在乳腺 X 光摄影中，传统上只关注单一指标（如平均腺体剂量）的辐射诱导癌症风险评估中，应该考虑这些结果。
