Parsons David, Zhang You, Gu Xuejun, Lu Weiguo
Department of Radiation Oncology, University of Texas, Southwestern Medical Center, 2280 Inwood Rd., Dallas, TX, 75390, USA.
Med Phys. 2020 Aug;47(8):3647-3657. doi: 10.1002/mp.14221. Epub 2020 May 25.
GammaPod, a stereotactic partial breast irradiator allowing highly conformal radiation dose delivery, has its unique mechanical design and treatment planning system (TPS). However, the uniqueness of the system poses challenges regarding initial GammaPod system commissioning and routine quality assurance (QA). In this study, we report POD-DOSI, a dedicated dosimetry system for accurate and efficient commissioning and QA of GammaPod.
The POD-DOSI system consists of two subsystems, POD-Scanner and POD-Calculator. The POD-Scanner is an automatic ion-chamber positioning system driven by two translational stepper motors for anterior-posterior, longitudinal, and lateral beam scanning. The stepper motors are controlled by a microcomputer through an in-house-developed graphical user interface, which can be remotely accessed by a laptop via wireless connection. The POD-Calculator is a commissioned GPU-based Monte Carlo dose calculation engine, which calculates dose by transporting particles from phase space constructed for GammaPod. In our institution, the POD-DOSI system was used for GammaPod TPS commissioning and dose verification. The POD-Calculator was further developed as a secondary dose calculation tool performing patient-specific plan QA before each treatment.
The POD-DOSI system has been fully evaluated and tested, both mechanically and dosimetrically, and applied successfully to drive the commissioning of our GammaPod system. The POD-Scanner achieved 0.1 mm accuracy in ion-chamber positioning tests. The POD-Calculator generated dose profiles matched well with water phantom measurements and TPS calculations to <0.5 mm accuracy. For end-to-end test on 56 different treatment plans, in-water point dose measurements by POD-Scanner were within ±2.20% of the doses calculated by POD-Calculator (range: -2.01% to 2.20%, mean: 0.04%, std_dev: 1.10%). Correspondingly, when switching the calculation medium from water to breast tissue, the point doses calculated by the POD-Calculator were within ±1.60% of the point doses calculated by the GammaPOD TPS (range: -1.59% to 1.51%, mean: -0.02%, std_dev: 0.73%). The average three-dimensional gamma passing rate between the GammaPod TPS doses and the POD-Calculator doses was 97.10% under the 2%/1 mm gamma criteria. The POD-DOSI system substantially shortened the GammaPod dosimetry commissioning time from weeks to days.
The developed POD-DOSI system resolves the challenges and streamlines the process of GammaPod commissioning and QA. It improves the efficiency and accuracy for both GammaPod commissioning and routine patient-specific QA.
GammaPod是一种立体定向部分乳腺照射器，能够实现高度适形的放射剂量输送，具有独特的机械设计和治疗计划系统（TPS）。然而，该系统的独特性给GammaPod系统的初始调试和常规质量保证（QA）带来了挑战。在本研究中，我们报告了POD - DOSI，这是一种用于GammaPod精确高效调试和QA的专用剂量测定系统。
POD - DOSI系统由两个子系统组成，即POD - 扫描仪和POD - 计算器。POD - 扫描仪是一个自动电离室定位系统，由两个平移步进电机驱动，用于前后、纵向和横向射束扫描。步进电机由一台微型计算机通过自行开发的图形用户界面控制，该界面可由笔记本电脑通过无线连接进行远程访问。POD - 计算器是一个基于GPU的已调试蒙特卡罗剂量计算引擎，它通过从为GammaPod构建的相空间传输粒子来计算剂量。在我们机构中，POD - DOSI系统用于GammaPod TPS调试和剂量验证。POD - 计算器进一步开发为二级剂量计算工具，在每次治疗前进行患者特异性计划QA。
POD - DOSI系统已在机械和剂量学方面进行了全面评估和测试，并成功应用于推动我们GammaPod系统的调试。POD - 扫描仪在电离室定位测试中实现了0.1毫米的精度。POD - 计算器生成的剂量分布与水模测量结果和TPS计算结果匹配良好，精度达到<0.5毫米。对于56个不同治疗计划的端到端测试，POD - 扫描仪在水中的点剂量测量值在POD - 计算器计算剂量的±2.20%范围内（范围：-2.01%至2.20%，平均值：0.04%，标准差：1.10%）。相应地，当将计算介质从水切换到乳腺组织时，POD - 计算器计算的点剂量在GammaPOD TPS计算的点剂量的±1.60%范围内（范围：-1.59%至1.51%，平均值：-0.02%，标准差：0.73%）。在2%/1毫米伽马标准下，GammaPod TPS剂量与POD - 计算器剂量之间的平均三维伽马通过率为97.10%。POD - DOSI系统将GammaPod剂量测定调试时间从数周大幅缩短至数天。
所开发的POD - DOSI系统解决了挑战，简化了GammaPod调试和QA的流程。它提高了GammaPod调试和常规患者特异性QA的效率和准确性。
