Performance limitations in feedback control systems with application to radiotherapy

外部放射治疗是一种使用外部高能x射线束根除肿瘤细胞的癌症治疗过程。最近，申请人和其他研究人员在放射治疗过程中引入了反馈控制，以便x射线束在肿瘤移动时跟踪肿瘤的位置；运动是由一系列过程引起的，包括呼吸、心跳和胃肠道的变化。反馈方案的优点包括减少副作用，更准确的放射剂量传递到肿瘤，并减少治疗时间。放射治疗的研究揭示了当前控制工程知识的不足，并启发了几个方向的研究，包括提出的研究方向。提议的研究试图回答的基本问题是：使用反馈控制能做什么有任何限制吗？了解这些性能限制的存在和本质是很重要的，因为这些知识可以帮助工程师避免试图解决不可能解决的问题，并帮助工程师修改问题以克服否则无法克服的性能限制。提出的研究将集中在控制器性能的几个方面。首先，“追踪”的问题得到了解决：x射线束追踪肿瘤的程度是否存在限制？其次，处理“干扰排斥”问题：如果患者在治疗台上移动，肿瘤跟踪系统是否有根本原因不能适应运动？最后，研究了“稳健性”问题：对患者肿瘤动态的了解必须达到多少才能使治疗有效？性能限制存在的几个潜在原因将被考虑，包括时间延迟、系统不稳定性和控制器结构。虽然受到放射治疗的激励并专门应用于放射治疗，但拟议研究中的理论工作是一般的；事实上，几乎所有使用反馈控制的系统（飞机、汽车、核反应堆、蒸馏塔、火箭等）都可能从这项工作中受益。

External radiotherapy is a cancer treatment process where external high-energy x-ray beams are used to eradicate tumor cells.  Recently, the applicant and other researchers have introduced feedback control into the radiotherapy process so that the x-ray beam tracks the position of the tumor as it moves; movement is caused by a number of processes, including breathing, heart beat, and changes in the gastrointestinal tract.  Advantages of the feedback scheme include a reduction in side effects, more accurate radiation dose delivery to the tumor, and a reduction in treatment time.  The radiotherapy research has revealed deficiencies in current control engineering knowledge, and has inspired research in several directions, including the direction of the proposed research.  The basic question that the proposed research seeks to answer is:  are there any limitations on what can be done using feedback control?  Understanding the existence and nature of such performance limitations is important because the knowledge helps engineers avoid trying to solve impossible problems, and aids engineers in modifying problems to overcome performance limitations that otherwise cannot be overcome.  The proposed research will focus on several aspects of controller performance.  First, the issue of "tracking" is addressed:  is there a limit to how well the x-ray beam can track the tumor?  Second, the issue of "disturbance rejection" is dealt with:  if the patient moves on the treatment table, is there any fundamental reason why the tumor tracking system should not be able to accommodate the movement?  Finally, the issue of "robustness" is studied:  how much knowledge about the patient's tumor dynamics must be known for treatment to be effective?   Several potential causes for the existence of performance limitations will be considered, including time delays, instabilities in the system, and controller structure.  Although motivated by and applied specifically to radiotherapy, the theoretical work in the proposed research is general; indeed, almost any system that uses feedback control (airplanes, automobiles, nuclear reactors, distillation columns, rockets, etc.) can potentially benefit from the work.