Radiation Medicine Program, Princess Margaret Hospital, University of Toronto, Toronto, Ontario M5G 2M9, Canada.
Med Phys. 2010 Oct;37(10):5322-9. doi: 10.1118/1.3488979.
The impact of photon beam energy and tissue heterogeneities on dose distributions and dosimetric characteristics such as point dose, mean dose, and maximum dose was investigated in the context of small-animal irradiation using Monte Carlo simulations based on the EGSnrc code.
Three Monte Carlo mouse phantoms, namely, heterogeneous, homogeneous, and bone homogeneous were generated based on the same mouse computed tomography image set. These phantoms were generated by overriding the tissue type of none of the voxels (heterogeneous), all voxels (homogeneous), and only the bone voxels (bone homogeneous) to that of soft tissue. Phase space files of the 100 and 225 kVp photon beams based on a small-animal irradiator (XRad225Cx, Precision X-Ray Inc., North Branford, CT) were generated using BEAMnrc. A 360 degrees photon arc was simulated and three-dimensional (3D) dose calculations were carried out using the DOSXYZnrc code through DOSCTP in the above three phantoms. For comparison, the 3D dose distributions, dose profiles, mean, maximum, and point doses at different locations such as the isocenter, lung, rib, and spine were determined in the three phantoms.
The dose gradient resulting from the 225 kVp arc was found to be steeper than for the 100 kVp arc. The mean dose was found to be 1.29 and 1.14 times higher for the heterogeneous phantom when compared to the mean dose in the homogeneous phantom using the 100 and 225 kVp photon arcs, respectively. The bone doses (rib and spine) in the heterogeneous mouse phantom were about five (100 kVp) and three (225 kVp) times higher when compared to the homogeneous phantom. However, the lung dose did not vary significantly between the heterogeneous, homogeneous, and bone homogeneous phantom for the 225 kVp compared to the 100 kVp photon beams.
A significant bone dose enhancement was found when the 100 and 225 kVp photon beams were used in small-animal irradiation. This dosimetric effect, due to the presence of the bone heterogeneity, was more significant than that due to the lung heterogeneity. Hence, for kV photon energies of the range used in small-animal irradiation, the increase of the mean and bone dose due to the photoelectric effect could be a dosimetric concern.
利用 EGSnrc 代码的蒙特卡罗模拟，研究光子束能量和组织非均匀性对小动物照射的剂量分布和剂量学特性（如点剂量、平均剂量和最大剂量）的影响。
基于同一组小鼠计算机断层扫描图像集，生成了三个蒙特卡罗小鼠体模，即不均匀体模、均匀体模和骨均匀体模。这些体模是通过将所有体素（均匀体模）或仅骨体素（骨均匀体模）的组织类型覆盖为软组织，而不是将所有体素（均匀体模）或仅骨体素（骨均匀体模）的组织类型覆盖为软组织，从而生成的。基于小动物辐照器（XRad225Cx，Precision X-Ray Inc.，North Branford，CT）的 100 和 225 kVp 光子束的相空间文件是使用 BEAMnrc 生成的。模拟了 360 度光子弧，并使用 DOSCTP 在上述三个体模中使用 DOSXYZnrc 代码进行了三维（3D）剂量计算。为了进行比较，在三个体模中确定了不同位置（等中心、肺、肋骨和脊柱）的 3D 剂量分布、剂量轮廓、平均值、最大值和点剂量。
发现 225 kVp 弧的剂量梯度比 100 kVp 弧更陡峭。与使用 100 和 225 kVp 光子束的均匀体模相比，不均匀体模的平均剂量分别高出 1.29 倍和 1.14 倍。与均匀体模相比，不均匀小鼠体模中的骨剂量（肋骨和脊柱）在 100 kVp 时约为 5 倍，在 225 kVp 时约为 3 倍。然而，与 100 kVp 光子束相比，225 kVp 光子束在不均匀、均匀和骨均匀体模中，肺剂量没有明显差异。
在小动物照射中使用 100 和 225 kVp 光子束时，发现骨剂量显著增加。这种由于骨非均匀性引起的剂量学效应比由于肺非均匀性引起的效应更为显著。因此，对于小动物照射中使用的范围内的 kV 光子能量，光电效应引起的平均剂量和骨剂量的增加可能是一个剂量学问题。
