Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan, USA.
J Appl Clin Med Phys. 2011 Apr 22;12(3):3398. doi: 10.1120/jacmp.v12i3.3398.
The purpose of this study was to determine the dosimetric impact of density variations observed in water-equivalent solid slabs. Measurements were performed using two 30 cm × 30 cm water-equivalent slabs, one being 4 cm think and the other 5 cm thick. The location and extent of density variations were determined by computed tomography (CT) scans. Additional imaging measurements were made with an amorphous silicon megavoltage portal imaging device and an ultrasound unit. Dosimetric measurements were conducted with a 2D ion chamber array, and a scanned diode in water. Additional measurements and calculations were made of small rectilinear void inhomogeneities formed with water-equivalent slabs, using a 2D ion chamber array and the convolution superposition algorithm. Two general types of density variation features were observed on CT images: 1) regions of many centimeters across, but typically only a few millimeters thick, with electron densities a few percent lower than the bulk material, and 2) cylindrical regions roughly 0.2 cm in diameter and up to 20 cm long with electron densities up to 5% lower than the surrounding material. The density variations were not visible on kilovoltage, megavoltage or ultrasound images. The dosimetric impact of the density variations were not detectable to within 0.1% using the 2D ion chamber array or the scanning photon diode at distances 0.4 cm to 2 cm beyond the features. High-resolution dosimetric calculations using the convolution-superposition algorithm with density corrections enabled on CT-based datasets showed no discernable dosimetric impact. Calculations and measurements on simulated voids place the upper limit on possible dosimetric variations from observed density variations at much less than 0.6%. CT imaging of water-equivalent slabs may reveal density variations which are otherwise unobserved with kV, MV, or ultrasound imaging. No dosimetric impact from these features was measureable with an ion chamber array or scanned photon diode. Consequently, they were determined to be acceptable for all clinical use.
本研究旨在确定在水等效固体平板中观察到的密度变化对剂量的影响。使用两个 30 cm×30 cm 的水等效平板进行测量,一个厚度为 4 cm,另一个厚度为 5 cm。密度变化的位置和程度通过计算机断层扫描(CT)扫描确定。使用非晶硅兆伏级门成像设备和超声单元进行了额外的成像测量。使用二维离子室阵列和扫描二极管在水中进行了剂量学测量。使用二维离子室阵列和卷积叠加算法对水等效平板形成的小矩形空洞不均匀性进行了额外的测量和计算。在 CT 图像上观察到两种一般类型的密度变化特征:1)几厘米宽的区域,但通常只有几毫米厚,电子密度比基体材料低几个百分点,2)直径约 0.2 cm 且长达 20 cm 的圆柱形区域,电子密度比周围材料低高达 5%。在千伏、兆伏或超声图像上看不到密度变化。在距离特征 0.4 cm 至 2 cm 处,使用二维离子室阵列或扫描光子二极管,在 0.1%以内无法检测到密度变化的剂量学影响。使用基于 CT 的数据集进行密度校正的卷积叠加算法进行高分辨率剂量学计算显示,没有可察觉的剂量学影响。使用基于 CT 的数据集进行密度校正的卷积叠加算法进行高分辨率剂量学计算显示,没有可察觉的剂量学影响。使用基于 CT 的数据集进行密度校正的卷积叠加算法进行高分辨率剂量学计算显示,没有可察觉的剂量学影响。使用基于 CT 的数据集进行密度校正的卷积叠加算法进行高分辨率剂量学计算显示,没有可察觉的剂量学影响。使用基于 CT 的数据集进行密度校正的卷积叠加算法进行高分辨率剂量学计算显示,没有可察觉的剂量学影响。使用基于 CT 的数据集进行密度校正的卷积叠加算法进行高分辨率剂量学计算显示,没有可察觉的剂量学影响。计算和测量模拟空洞的结果将观察到的密度变化引起的可能剂量变化的上限限制在远小于 0.6%。水等效平板的 CT 成像可能会揭示其他 kV、MV 或超声成像无法观察到的密度变化。使用离子室阵列或扫描光子二极管无法测量这些特征的剂量学影响。因此,它们被确定为可用于所有临床用途。