基于蒙特卡罗计算的伽玛刀剂量学研究

发布时间：2018-08-19 11:29

【摘要】：论文以我国自行研制并已投入临床使用的LunaTM-260全身伽玛刀治疗机为对象,研究了该伽玛刀单源和多源的静态聚焦的剂量学特性。目的,建立Luna伽玛刀的蒙特卡罗剂量计算模型,模拟计算伽玛刀剂量学参数。解决Luna伽玛刀在临床使用中单源的百分深度剂量(PDD)、组织最大剂量比(TMR)、射野离轴比(OAR)及输出因子等剂量学参数无法测量的问题,并将蒙卡计算结果与治疗计划系统的数据比较,对验证伽玛刀治疗的剂量准确性进行验证。课题涉及伽玛刀机头的几何建模、胶片剂量计的实验测量、蒙特卡罗程序的模拟计算等工作。论文介绍并论述了目前伽玛刀计划系统的剂量计算模型及优缺点以及蒙特卡罗方法的基本原理。着重论述了Luna伽玛刀计划系统采用的TMR-OAR剂量计算模型,涉及到的一些剂量学参数,讨论了蒙特卡罗程序MCNP4C对开展本研究的可行性和实用性。详述了基于Luna伽玛刀的机头结构、材料和尺寸,以及放射源的特征的MCNP剂量计算模型的几何建模。并通过与测量数据的比较来验证计算模型的正确性,基于所建剂量计算模型开展伽玛刀单源和多源聚焦的剂量学研究。伽玛刀的实验测量采用Gafchromic EBT2新型胶片剂量计。研究方法：通过建立伽玛刀剂量计算模型,分别计算单源和多源射束所有准直器在30cm×30cm×30cm水模体中的剂量参数和在直径16cm的伽玛刀专用有机玻璃球模中的等剂量分布。结果：模拟计算了luna伽玛刀6种准直器(4mm×4mm、8mm×8mm、14mm×14mm、14mm×20mm、14mm×40mm、14mm×60mm)单源和多源的在水模中的0.5cm、5cm及10cm深度出处的百分深度剂量及离轴比；计算了在专用测量球模中的等剂量分布曲线。在水模0.5cm深度处4、5、6号准直器的射野离轴比计算结果不好；5cm和10cm处的计算结果比较理想。根据百分深度剂量与组织最大剂量比的关系,由百分深度剂量数据得到组织最大剂量比的值。将单源方模中的组织最大剂量比的计算结果、计划系统中的原始数据(测量数据)及标准的组织最大剂量比曲线进行了比较：计算结果与标准曲线符合的较好,与TPS中数据(测量)有一定误差,最大误差不超过5%。对于l号准直器在0—5cm深度范围计算值比测量值大,5—15cm深度范围计算值与测量值基本一致,大于15cm的深度计算值比测量值小；对于其余的5个准直器计算值与测量值一致,测量值大于计算值,误差为2%-5%。单源方模中5cm深度处射野离轴比的比较结果：射野内的数值符合的较好,射野外的数值相差较大。每个准直器取其焦点处的剂量作为标准值,以6号准直器最为标准归一计算了6个准直器的输出因子,将其与测量结果比较。1号准直器相差较大(大于10%),其余均小于3%。结论：所建立的蒙特卡罗计算模型很好地验证了单源射野的百分深度剂量(PDD)、组织最大剂量比(TMR)、射野离轴比(OAR)及输出因子,计算结果与测量结果在误差范围(5%)内符合的很好；同时,对于多源聚焦的剂量分布特点进行了研究,给出luna伽玛刀的焦点剂量分布。研究结果表明所建立的luna伽玛刀蒙特卡罗剂量计算模型可以用于其剂量学参数的验证,很好地解决了单源剂量学参数无法测量的问题,同时对于伽玛刀的临床上计划设计和剂量计算起到一定的指导性作用。本工作为伽玛刀今后研究工作奠定了基础。
[Abstract]:The dosimetric characteristics of single-source and multi-source static focusing of LunaTM-260 gamma knife were studied. Objective To establish a Monte Carlo dosimetric model of Luna gamma knife and simulate the dosimetric parameters of Luna gamma knife. The dosimetric parameters such as PDD, TMR, OAR and output factor can not be measured. The results of Monte Carlo simulation were compared with the data of treatment planning system to verify the dose accuracy of gamma knife therapy.
The subject involves the geometric modeling of gamma knife head, the experimental measurement of film dosimeter and the simulation calculation of Monte Carlo program. The paper introduces and discusses the dose calculation model, advantages and disadvantages of the current gamma knife planning system and the basic principle of Monte Carlo method. The TMR-OAR used in Luna gamma knife planning system is emphatically discussed. The feasibility and practicability of the Monte Carlo program MCNP4C for this study are discussed. The geometric modeling of the MCNP dose calculation model based on Luna gamma knife is described in detail, including the structure, material and size, and the characteristics of the radiation source. Based on the correctness of the calculation model, single-source and multi-source focused gamma knife dosimetry was carried out. Gafchromic EBT2 film dosimeter was used in the experimental measurement of gamma knife.
Research methods: By establishing gamma knife dose calculation model, the dose parameters of all collimators of single source and multi-source beam in 30 cm 65507
Results: The percentile depth doses and off-axis ratios of 6 kinds of Luna gamma knife collimators (4 mm 4 mm, 8 mm 8 mm, 14 mm 14 mm, 14 mm 65 According to the relationship between the percentage depth dose and the maximum dose ratio of tissue, the maximum dose ratio of tissue can be obtained from the percentage depth dose data. Comparing the measured data with the standard tissue maximum dose ratio curve, the calculated results are in good agreement with the standard curve, and the maximum error is less than 5% with the TPS data. The calculated depth at 15 cm is smaller than the measured value, and the measured value is larger than the calculated value for the remaining 5 collimators, with an error of 2%-5%. The comparison of the off-axis ratio of the field at the depth of 5 cm in a single source square model shows that the values in the field are in good agreement with those in the field, and the values in the field are quite different. The output factors of the six collimators were normalized by using collimator No. 6 as the standard value. The difference between collimator No. 1 and the measured results was larger (more than 10%) and the rest was less than 3%.
CONCLUSION: The Monte Carlo calculation model validates the PDD, TMR, OAR and output factor of single source radiation field. The calculated results are in good agreement with the measured results within the error range (5%). The results show that the Monte Carlo dose calculation model of Luna gamma knife can be used to validate the dosimetric parameters of Luna gamma knife. It can solve the problem that single source dosimetric parameters can not be measured. It also plays a guiding role in clinical planning and dose calculation of Luna gamma knife. This work lays the foundation for future research work of gamma knife.
【学位授予单位】：中国人民解放军军事医学科学院
【学位级别】：博士
【学位授予年份】：2012
【分类号】：R142

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