基于横向非均匀修正的解析质子剂量计算方法研究

Radiation treatment planning system (TPS) is an intelligent software system to plan the radiotherapy before treatment. Dose calculation is the core of TPS, and its precision directly affects the effect of radiotherapy. Because the characteristic of dose deposition of Bragg peak can greatly reduce the irradiated dose on human’s normal organ, proton therapy has been applied more than ever before. However, because of the gradient distribution near the area of Bragg peak, the accuracy of dose calculation is more highly demanded. So far, the popular pencil beam algorithm lacks enough consideration of the lateral inhomogeneity of tissue. The algorithm only divides proton beam to small beamlets to reduce the anisotropic impact. But due to the limitation of computation time, the inhomogeneity of the beamlet will significantly decrease the accuracy of results. The work of this proposed project obtains the physical parameters of the pencil beam algorithm by using the Monte Carlo simulation and experiments. The project mainly focuses on solving the key problems which involved in the lateral inhomogeneous corrections of human tissues in analytical proton dose calculation, and meanwhile develops the self-adaptive method of dividing the sub-beam based on regular polygon proton beam, and also develops the equivalent depth model based on the 3-D inhomogeneous corrections in the sub-beam. Moreover, the project can reveal the law of inhomogeneous corrections how to affect the accuracy of proton dose calculation, and develop the corresponding algorithm of dose calculation. Thus, this project can finally lay the foundations of the dose engine of proton therapy which owns China's independent intellectual property rights.

放射治疗计划系统是制定放疗计划的智能化软件系统，剂量计算作为其核心，直接影响治疗效果。质子放疗因其布拉格峰的剂量沉积特性能显著降低正常器官受照剂量，受到越来越多的应用，但布拉格峰高梯度分布也对剂量计算的精确性提出了更高的要求。目前广泛使用的解析笔形束剂量计算方法对组织横向非均匀性的考虑不足，只是将质子束分解成子束，以减小非均匀性的影响。但受到计算时间的限制，无法细分的子束内的非均匀性会明显影响计算结果的精确性。本课题利用蒙特卡罗模拟和实验测量等手段得到笔形束方法所需的物理参数，重点围绕实现基于横向组织非均匀性修正的解析质子剂量计算方法中所涉及的关键科学问题展开研究，提出基于正多边形的质子束自适应分解子束方法和基于子束内三维非均匀修正的等效深度模型，揭示组织横向非均匀性对解析质子剂量计算精度影响的规律，并发展了对应的剂量计算方法，为开发具有我国自主知识产权的精确质子放疗剂量计算引擎奠定基础。

质子放射治疗因其布拉格峰的剂量沉积特性能显著降低正常器官受照剂量，因而得到越来越广的应用，但布拉格峰高梯度分布的特点也对剂量计算的精确性提出了更高的要求。现有使用的解析笔形束剂量计算方法对组织的横向非均匀性考虑不足，只是将质子束分解成子束，以减小非均匀性的影响。但受到计算时间的限制，无法细分的子束内的非均匀性会影响计算结果的精确性。本项目利用蒙特卡罗模拟和实验测量等手段得到笔形束方法所需的物理参数，重点围绕实现基于横向组织非均匀性修正的解析质子剂量方法中所涉及的关键科学问题展开研究：（1）研究了高效自适应的笔形束分解方法，适用于多种质子放疗方式和不同束流参数，能在保证精度的前提下，提高分解效率，从而提高计算效率；（2）开发了基于横向非均匀性修正的笔形束剂量模型，能很好地处理笔形束路径上及横向上的非均匀性，通过分析其对质子射程、散射的影响从而得到更准确的三维剂量分布。计算结果表明：剂量计算方法/模型的结果与TOPAS结算结果符合较好，真实病例的γ通过率(2%/2mm)不低于97.6%。发展的关键方法与技术，可以用于精确质子放射治疗中的剂量计算中提高剂量计算的精度，提高肿瘤患者的治愈率。