基于蒙特卡罗计算的伽玛刀剂量学研究

【作者】 王军良；

【导师】 杨国山；

【作者基本信息】 中国人民解放军军事医学科学院 ， 放射医学， 2012， 博士

【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。更多还原

【Abstract】 The paper’s object of study is the LunaTM-260whole body gamma knife treatm ent machine. Luna has developed by China and has been used in clinical.The pape r studyed the dosimetric properties of the Gamma Knife single-source and multi-so urces static focus. Purpose to setup the Gamma Knife Luna Monte Carlo dose cal culation model, and simulationly caclulate Gamma Knife dosimetry parameters. To solve the problem that the Luna Gamma Knife in the clinical use of single-source percentage depth dose (PDD), tissue maximum dose ratio (TMR), beam off-axis r atio (OAR), and dosimetric parameters such as output factors can not be measured. Compared Monte Carlo calculation resluts and treatment planning system data to verify the dose accuracy of Gamma Knife treatment for patients..Works involved the Gamma Knife head of geometric modeling and experimenta1measurements of film dosimeter and Monte Carlo simulation program. This paper introduces and discusses the Gamma Knife planning system for dose calculation model and their the advantage and disadvantage and the basic principle of the Mo nte Carlo method. Focus on the TMR-OAR dose calculation model use for the L una Gamma Knife planning system, and the feasibility and practicality of the Mon te Carlo code of MCNP4C for carrying out this study are discussed. Details the basic structure of the gamma knife-based Luna, the structure and size of the head section, the various parts of the material and radioactive sources, structure, shape MCNP dose calculation model of the modeling process. With the measurement dat a to verify the correctness of the calculation model, make sure built a computation al model correctly, the dosimetric parameters of the Gamma Knife single-source an d multi-source simulation and measurement data which is getted by Gafchromic E BT2new film dosimetry..Methods:Based on the Gamma Knife dose calculation model, I calculated dose parameters of the single-source and multi-source for all beam collimators in the30cm×30cm×30cm water phantom and the dose distribution the in the diamete r of16cm Gamma Knife dedicated plexiglass ball. Results:Percentage depth dose and off-axis are calculated for luna Gamma Kni fe6collimators (4mm×4mm,8mm×8mm,14mm×14mm,14mm×20mm,14mm x40mm,14mm x60mm) single-source and multi-source0.5cm,5cm an d10cm depth of the source in the water phantom and dose distribution curve in t he special ball modle. In0.5cm depth of the phantom, calculation results of the N o.4、No.5、No.6collimator of beam off-axis are not good; calculation results are g ood in5cm and10cm of the phantom. TMR(Tissue Maximum Ratio) value are c alculated from the PDD(percentage depth dose) according to the relationship betw een the PDD and the TMR. For single-source TMR calculation results, the raw da ta (measurements) in the planning system and the standard curve are compared, th e calculation results are well consistent with the standard curve, and There is a certain error between the calculation results and TPS data (measurement), which is less than5%. For the No.l collimator calculated values are more than the measur ed value from0to5cm depth in the modle, from5cm to15cm depth,the calcul ated values and measured values are basically the same, More than15cm depth,cal culated values are less than the measured values; For5the rest the collimators cal culate the values are consistent with the measured values, the measured value is g reater than the calculated value, an error is about2%-5%. In5cm depth square p hantom, beam off-axis comparison results of the Single-source:the data within the radiation field is good, but the data is not good outside the field. Each collimat or whichever is the focus at the dose as a standard value, the collimator on the N o.6most standard normalized calculated6collimator output factor and its comparis on with the measurement results. For the No.1collimator (greater than10%), and the rest were less than3%.Conclusion:Monte Carlo calculation model is well validated single-source beam percentage depth dose (PDD), the maximum dose ratio (TMR), the beam off-axis r atio (OAR) and output factor. Calculationthe results accord with measurement resul ts within the error range (5%) and the focus specialty of multi-source dose distrib ution was studied, the luna Gamma Knife dose distribution is presented. The result s show that the the luna Gamma Knife Monte Carlo dose calculation model can b e used for the validation of dosimetric parameters which has been founded. It ver y well soleved the problem that a single-source dosimetric parameters can not be measured.,while It can guide the Gamma Knife clinical plan design and dose calcu lation.This work laid the foundation for the Gamma Knife in the future research w ork更多还原