Dipartimento di Fisica, Alma Mater Studiorum, Universita' di Bologna, Viale Berti-Pichat 6/2, 40127 Bologna, Italy.
Phys Med. 2013 Jun;29(4):379-87. doi: 10.1016/j.ejmp.2012.06.001. Epub 2012 Jul 2.
Cone-beam breast Computed Tomography (bCT) is an X-ray imaging technique for breast cancer diagnosis, in principle capable of delivering a much more homogeneous dose spatial pattern to the breast volume than conventional mammography, at dose levels comparable to two-view mammography. We present an investigation of the three-dimensional dose distribution for a cone-beam CT system dedicated to breast imaging. We employed Monte Carlo simulations for estimating the dose deposited within a breast phantom having a hemiellipsoidal shape placed on a cylinder of 3.5 cm thickness that simulates the chest wall. This phantom represents a pendulant breast in a bCT exam with the average diameter at chest wall, assumed to correspond to a 5-cm-thick compressed breast in mammography. The phantom is irradiated in a circular orbit with an X-ray cone beam selected from four different techniques: 50, 60, 70, and 80 kVp from a tube with tungsten anode, 1.8 mm Al inherent filtration and additional filtration of 0.2 mm Cu. Using the Monte Carlo code GEANT4 we simulated a system similar to the experimental apparatus available in our lab. Simulations were performed at a constant free-in-air air kerma at the isocenter (1 μGy); the corresponding total number of photon histories per scan was 288 million at 80 kVp. We found that the more energetic beams provide a more uniform dose distribution than at low energy: the 50 kVp beam presents a frequency distribution of absorbed dose values with a coefficient of variation almost double than that for the 80 kVp beam. This is confirmed by the analysis of the relative dose profiles along the radial (i.e. parallel to the "chest wall") and longitudinal (i.e. from "chest wall" to "nipple") directions. Maximum radial deviations are on the order of 25% for the 80 kVp beam, whereas for the 50 kVp beam variations around 43% were observed, with the lowest dose values being found along the central longitudinal axis of the phantom.
锥形束乳腺计算机断层摄影术（bCT）是一种用于乳腺癌诊断的 X 射线成像技术，原则上能够比传统的乳房 X 光摄影术向乳房体积提供更均匀的剂量空间分布，同时剂量水平与双视角乳房 X 光摄影术相当。我们介绍了一种专门用于乳腺成像的锥形束 CT 系统的三维剂量分布研究。我们采用蒙特卡罗模拟方法来估算放置在模拟胸壁的 3.5cm 厚圆柱上的具有半椭圆形形状的乳腺体模内沉积的剂量。该体模代表了 bCT 检查中摆动乳房的情况，其平均直径在胸壁处，假定与乳房 X 光摄影术的 5cm 厚压缩乳房相对应。该体模在具有钨阳极的管中选择的 X 射线锥形束的圆形轨道上进行照射，从 50、60、70 和 80kVp 中选择四个不同的技术：管中有 1.8mm Al 固有过滤和 0.2mm Cu 的额外过滤。我们使用蒙特卡罗代码 GEANT4 模拟了与我们实验室中可用的实验设备类似的系统。在等中心处以恒定的自由空气比释动能（1μGy）进行模拟；在 80kVp 时，每个扫描的光子历史总数为 2.88 亿次。我们发现，能量较高的射线束比低能量的射线束提供更均匀的剂量分布：50kVp 射线束的吸收剂量值的频率分布的变异系数几乎是 80kVp 射线束的两倍。这一点通过对径向（即平行于“胸壁”）和纵向（即从“胸壁”到“乳头”）方向的相对剂量分布的分析得到了证实。80kVp 射线束的最大径向偏差约为 25%，而 50kVp 射线束的偏差约为 43%，在体模的中央纵向轴上发现了最低的剂量值。
