Rampado Osvaldo, Giglioli Francesca Romana, Rossetti Veronica, Fiandra Christian, Ragona Riccardo, Ropolo Roberto
Struttura Complessa Fisica Sanitaria, Azienda Ospedaliero Universitaria Città della Salute e della Scienza, Corso Bramante 88, Torino 10126, Italy.
Radiation Oncology Department, University of Turin, Torino 10126, Italy.
Med Phys. 2016 May;43(5):2515. doi: 10.1118/1.4947129.
The aim of this study was to evaluate various approaches for assessing patient organ doses resulting from radiotherapy cone-beam CT (CBCT), by the use of thermoluminescent dosimeter (TLD) measurements in anthropomorphic phantoms, a Monte Carlo based dose calculation software, and different dose indicators as presently defined.
Dose evaluations were performed on a CBCT Elekta XVI (Elekta, Crawley, UK) for different protocols and anatomical regions. The first part of the study focuses on using pcxmc software (pcxmc 2.0, STUK, Helsinki, Finland) for calculating organ doses, adapting the input parameters to simulate the exposure geometry, and beam dose distribution in an appropriate way. The calculated doses were compared to readouts of TLDs placed in an anthropomorphic Rando phantom. After this validation, the software was used for analyzing organ dose variability associated with patients' differences in size and gender. At the same time, various dose indicators were evaluated: kerma area product (KAP), cumulative air-kerma at the isocenter (Kair), cone-beam dose index, and central cumulative dose. The latter was evaluated in a single phantom and in a stack of three adjacent computed tomography dose index phantoms. Based on the different dose indicators, a set of coefficients was calculated to estimate organ doses for a range of patient morphologies, using their equivalent diameters.
Maximum organ doses were about 1 mGy for head and neck and 25 mGy for chest and pelvis protocols. The differences between pcxmc and TLDs doses were generally below 10% for organs within the field of view and approximately 15% for organs at the boundaries of the radiation beam. When considering patient size and gender variability, differences in organ doses up to 40% were observed especially in the pelvic region; for the organs in the thorax, the maximum differences ranged between 20% and 30%. Phantom dose indexes provided better correlation with organ doses than Kair and KAP, with average ratios ranging between 0.9 and 1.1 and variations for different organs and protocols below 20%. The triple phantom setup allowed us to take into account scatter dose contributions, but nonetheless, the correlation with the evaluated organ doses was not improved with this method.
The simulation of rotational geometry and of asymmetric beam distribution by means of pcxmc 2.0 enabled us to determine patient organ doses depending on weight, height and gender. Alternatively, the measurement of an in phantom dose indicator combined with proper correction coefficients can be a useful tool for a first dose estimation of in-field organs. The data and coefficients provided in this study can be applied to any patient undergoing a scan by an Elekta XVI equipment.
本研究的目的是通过在人体模型中使用热释光剂量计（TLD）测量、基于蒙特卡罗的剂量计算软件以及目前定义的不同剂量指标，评估多种用于评估放射治疗锥形束CT（CBCT）所致患者器官剂量的方法。
针对不同的扫描方案和解剖区域，在Elekta XVI型CBCT（英国克劳利的Elekta公司）上进行剂量评估。研究的第一部分重点是使用pcxmc软件（芬兰赫尔辛基的STUK公司的pcxmc 2.0）来计算器官剂量，以适当方式调整输入参数以模拟照射几何形状和射束剂量分布。将计算出的剂量与放置在人体Rando模型中的TLD读数进行比较。经过此验证后，该软件用于分析与患者体型和性别差异相关的器官剂量变异性。同时，评估了各种剂量指标：比释动能面积乘积（KAP）、等中心处的累积空气比释动能（Kair）、锥形束剂量指数和中心累积剂量。后者在单个模型以及三个相邻的计算机断层扫描剂量指数模型堆叠中进行了评估。基于不同的剂量指标，计算了一组系数，以使用其等效直径来估计一系列患者体型的器官剂量。
头部和颈部方案的最大器官剂量约为1 mGy，胸部和骨盆方案的最大器官剂量约为25 mGy。对于视野内的器官，pcxmc计算的剂量与TLD测量的剂量之间的差异通常低于10%，对于辐射束边界处的器官，差异约为15%。考虑患者体型和性别变异性时，观察到器官剂量差异高达40%，尤其是在骨盆区域；对于胸部器官，最大差异在20%至30%之间。模型剂量指数与器官剂量的相关性比Kair和KAP更好，平均比值在0.9至1.1之间，不同器官和方案的变化低于20%。三个模型的设置使我们能够考虑散射剂量的贡献，但尽管如此，这种方法与评估的器官剂量之间的相关性并未得到改善。
借助pcxmc 2.0对旋转几何形状和不对称射束分布进行模拟，使我们能够根据体重、身高和性别确定患者的器官剂量。或者，测量模型中的剂量指标并结合适当的校正系数，可成为对视野内器官进行首次剂量估计的有用工具。本研究中提供的数据和系数可应用于任何接受Elekta XVI设备扫描的患者。
