Bostani Maryam, McMillan Kyle, DeMarco John J, Cagnon Chris H, McNitt-Gray Michael F
Departments of Biomedical Physics and Radiology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California 90024.
Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, California 90095.
Med Phys. 2014 Nov;41(11):112101. doi: 10.1118/1.4887807.
Monte Carlo (MC) simulation methods have been widely used in patient dosimetry in computed tomography (CT), including estimating patient organ doses. However, most simulation methods have undergone a limited set of validations, often using homogeneous phantoms with simple geometries. As clinical scanning has become more complex and the use of tube current modulation (TCM) has become pervasive in the clinic, MC simulations should include these techniques in their methodologies and therefore should also be validated using a variety of phantoms with different shapes and material compositions to result in a variety of differently modulated tube current profiles. The purpose of this work is to perform the measurements and simulations to validate a Monte Carlo model under a variety of test conditions where fixed tube current (FTC) and TCM were used.
A previously developed MC model for estimating dose from CT scans that models TCM, built using the platform of mcnpx, was used for CT dose quantification. In order to validate the suitability of this model to accurately simulate patient dose from FTC and TCM CT scan, measurements and simulations were compared over a wide range of conditions. Phantoms used for testing range from simple geometries with homogeneous composition (16 and 32 cm computed tomography dose index phantoms) to more complex phantoms including a rectangular homogeneous water equivalent phantom, an elliptical shaped phantom with three sections (where each section was a homogeneous, but different material), and a heterogeneous, complex geometry anthropomorphic phantom. Each phantom requires varying levels of x-, y- and z-modulation. Each phantom was scanned on a multidetector row CT (Sensation 64) scanner under the conditions of both FTC and TCM. Dose measurements were made at various surface and depth positions within each phantom. Simulations using each phantom were performed for FTC, detailed x-y-z TCM, and z-axis-only TCM to obtain dose estimates. This allowed direct comparisons between measured and simulated dose values under each condition of phantom, location, and scan to be made.
For FTC scans, the percent root mean square (RMS) difference between measurements and simulations was within 5% across all phantoms. For TCM scans, the percent RMS of the difference between measured and simulated values when using detailed TCM and z-axis-only TCM simulations was 4.5% and 13.2%, respectively. For the anthropomorphic phantom, the difference between TCM measurements and detailed TCM and z-axis-only TCM simulations was 1.2% and 8.9%, respectively. For FTC measurements and simulations, the percent RMS of the difference was 5.0%.
This work demonstrated that the Monte Carlo model developed provided good agreement between measured and simulated values under both simple and complex geometries including an anthropomorphic phantom. This work also showed the increased dose differences for z-axis-only TCM simulations, where considerable modulation in the x-y plane was present due to the shape of the rectangular water phantom. Results from this investigation highlight details that need to be included in Monte Carlo simulations of TCM CT scans in order to yield accurate, clinically viable assessments of patient dosimetry.
蒙特卡罗（MC）模拟方法已广泛应用于计算机断层扫描（CT）中的患者剂量测定，包括估算患者器官剂量。然而，大多数模拟方法仅经过有限的验证，通常使用具有简单几何形状的均匀体模。随着临床扫描变得更加复杂且管电流调制（TCM）在临床上的使用日益普遍，MC模拟应在其方法中纳入这些技术，因此也应使用具有不同形状和材料组成的各种体模进行验证，以产生各种不同调制的管电流分布。本研究的目的是在使用固定管电流（FTC）和TCM的各种测试条件下进行测量和模拟，以验证蒙特卡罗模型。
使用先前开发的基于mcnpx平台构建的用于模拟TCM的CT扫描剂量估算MC模型进行CT剂量定量。为了验证该模型对FTC和TCM CT扫描患者剂量准确模拟的适用性，在广泛的条件下对测量值和模拟值进行了比较。用于测试的体模范围从具有均匀组成的简单几何形状（16厘米和32厘米计算机断层扫描剂量指数体模）到更复杂的体模，包括矩形均匀水等效体模、具有三个部分的椭圆形体模（每个部分是均匀但不同的材料）以及非均匀、复杂几何形状的人体模型体模。每个体模需要不同程度的x、y和z调制。每个体模在多排探测器CT（Sensation 64）扫描仪上在FTC和TCM条件下进行扫描。在每个体模内的不同表面和深度位置进行剂量测量。对每个体模使用FTC、详细的x - y - z TCM和仅z轴TCM进行模拟以获得剂量估计值。这使得能够在体模、位置和扫描的每种条件下对测量剂量值和模拟剂量值进行直接比较。
对于FTC扫描，所有体模测量值与模拟值之间的均方根（RMS）差异百分比在5%以内。对于TCM扫描，使用详细TCM和仅z轴TCM模拟时测量值与模拟值之间差异的RMS百分比分别为4.5%和13.2%。对于人体模型体模，TCM测量值与详细TCM和仅z轴TCM模拟之间的差异分别为1.2%和8.9%。对于FTC测量和模拟，差异的RMS百分比为5.0%。
本研究表明，所开发的蒙特卡罗模型在包括人体模型体模在内的简单和复杂几何形状下，测量值与模拟值之间具有良好的一致性。这项工作还表明，仅z轴TCM模拟的剂量差异增加，由于矩形水体模的形状，在x - y平面存在相当大的调制。本研究结果突出了在TCM CT扫描的蒙特卡罗模拟中需要纳入的细节，以便对患者剂量测定进行准确、临床可行的评估。
