基于笔射束核阵列的三维适形放射治疗剂量计算方法研究
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摘要
肿瘤放射治疗学与肿瘤外科学、肿瘤内科学一起构成恶性肿瘤治疗的三大主要手段,约有60%-70%的肿瘤病人在不同的治疗阶段需要接受放射治疗。放射治疗的目的是达到治疗的最优化,即靶区所受剂量尽可能大,而正常组织和危及器官尽可能少受或免受照射。计算机三维放射治疗计划系统是当今放射治疗的灵魂。本文针对放射治疗计划系统中的剂量计算模型进行了系统而深入的研究。
伴随着放射肿瘤学的发展,关于精确放射治疗技术的研究从未停止过。1949年,瑞典科学家Leksell首先提出放射外科学的构想,利用立体定向技术,使用大剂量的射束一次性摧毁需治疗的病灶,此后这项治疗方法被命名为立体定向放射外科(SRS),并在1955年应用于临床。上世纪60年代初期,日本科学家用一种电机驱动光栅控制放射线束的形状,使得射束的截面正好吻合病灶的投影。70年代初期,美国科学家开始认识到调强放疗(IMRT)的重要性并对其进行了初步研究。80年代末90年代初,美、德等国家相继开发出了商用的X刀；瑞典开发了第三代γ刀系统；同时期带有机械手的"Cyber刀”出现。1995年Spirou等人提出了使用动态多叶光栅来实现调强放疗,并用其治疗前列腺癌。进入新世纪以来,随着放射物理学、放射生物学、临床肿瘤学和医学影像学等相关学科的发展,放射治疗技术领域发生了巨大变革。精确放射治疗技术就是以“精确定位、精确计划、精确治疗”为特征的新的放射治疗技术的统称。精确放射治疗技术使得在肿瘤治疗上实现高精度、高剂量、高疗效和低损伤(三高一低)的现代放疗模式成为可能。
精确放射治疗技术较之常规放射治疗技术有很大的进步,主耍表现在：
(1)精确放射治疗技术一般采用CT、MRI、PET等先进的影像技术作为确定靶区与重要器官的依据,而常规放射治疗技术一般采用X光模拟定位图象；
(2)精确放射治疗技术一般具有三维治疗计划系统(TPS),需对影像数据进行三维重建和三维显示,并在此基础上进行三维计划设计,显示三维剂量分布,而常规放射治疗技术一般只能进行二维治疗计划设计；
(3)精确放射治疗技术一般采用面膜、真空负压垫、头环等固定装置结合三维坐标系统,可对病灶进行单次或多次重复的准确定位,而常规放射治疗技术一般采用体表勾画病灶等比较粗糙的定位方法；
(4)精确放射治疗技术一般采用多个非共面照射野或多个非共面照射弧的照射方式,而常规放射治疗技术一般采用单野照射或两野、三野照射；
(5)精确放射治疗技术可使高剂量区域与靶区在三维形状上更加一致,并保证靶区内及表面的剂量处处相等。也即实现剂量上的适形,常规照射治疗技术则无法达到。
剂量计算的结果作为评价治疗计划的依据,同时也是治疗计划优化与逆向治疗计划设计的基础。在计算机放射治疗计划系统迅速发展的同时,其核心内容之一的剂量计算模型和方法也在不断地发展。这种发展主要表现为从基于修正的剂量计算模式向基于模型的剂量计算模式的转变以及二者的相互渗透。基于修正的剂量计算模式是以参考条件下标准野的实验测量深度剂量、离轴比、散射因子等为基础,加之必要修正后得到实际放射物理条件和放射治疗对象剂量分布的剂量计算方法。随着计算机存储容量和运算速度的不断提高,其它更高精度的剂量计算方法不断地被用于计算机放射治疗计划系统中。这类计算方法一般以建立新的剂量计算模型为着眼点,全局考虑放射线与人体的相互作用,从而使基于理论计算的剂量分布与基于实验测量的剂量分布更吻合。文中对这两大类方法进行了较为详细的介绍,特别是集中对几种基于模型的剂量计算方法进行了详细的分析和研究。其中包括：将模体中任意一点的剂量分成原射线剂量和散射线剂量两部分的剂量计算方法被称作原射线剂量和散射线剂量分离法。此类计算包括了Clarkson法、微分散射空气比法、Day氏法等。
相比于上述几种方法,学术界普遍认为模特卡罗法为目前剂量计算的金标准。该方法进行剂量计算的原理为用统计学方法来模拟大量单能光子在输运过程中与物质的作用。光子进人某种介质(如人体组织)后,通过与介质中原子的相互作用,而传递电离辐射的部分或全部能量。该方法的先进之处主要表现在它能适用于任意几何形状的体模和各种次级粒子(如光子辐射产生的电子)上。
另一个比较精确的剂量计算模型是卷积法。在这种模型中,剂量计算是通过将放射线的光通分布与一个点扩展函数或卷积核相卷积来实现的。卷积法适用于任意的射野光通分布,所以这种方法特别适宜于不规则野的计算。另外,因为卷积可通过快速傅立叶变换来完成,这使卷积运算速度得到了显著地提高。
通过对上述几种基于模型的剂量计算方法进行了分析,选定采用卷积模型完成剂量计算,并且对该模型的原理进行了深入的研究,在该方法已有的理论基础上进行了改进,提高了模型的计算精确度。
卷积模型作为当前应用于放射治疗计划系统中的主流计算模型之一。当暂不考虑卷积核随深度的变化情况时,简化为平面内的注量分布与笔射束核的卷积。通过计算可以得到某一深度处于射束方向垂直的平面内的剂量分布。对于卷积运算中所需要用到的笔射束核,根据Fermi理论,在小角度散射的情况下,可以表示为两个相互独立的正交方向上的一维笔射束核的乘积形式。
一维笔射束核可以通过实际测量的宽平行束的剂量分布数据求解得到。再利用笔射束核重建各种形状射野的剂量分布,具有一定的计算精度的优势,减少了额外工作量。该方法的优点是：测量数据综合了照射过程的各种散射因素,模型不存在理想化和简化的问题。利用笔射束核重建测量野自身或受照条件与之相近的射野的剂量分布效果较好。但采用单一笔射束核重建任意射野剂量分布仍存在一定问题。
进入模体内的光子与介质相互作用产生次级电子将能量传递给介质,在建成区附近达到电子平衡。卷积模型中假设笔射束核具有空间不变性的这一前提,对直线加速器而言在达到电子平衡的区域外难以得到保证。在直线加速器的剂量分布中,散射线的剂量贡献占有极其重要的一部分。均匀模体中散射线剂量受射野大小和深度,以及同一方向上准直器的近端和远端边界的影响,其变化较为复杂。在较大深度处,模体内散射占主要；较浅深度处,受准直器散射影响较多。由于各种干扰因素的存在,造成射线能谱发生改变,使实际笔射束核的剂量分布并非是一个空间不变量,受到笔射束核为空间不变量这一假设的限制,对其进行修正,会减弱卷积模型可以通过快速傅里叶变换完成的这一优点。利用单一的笔射束核对任意射野的剂量分布进行重建,因为对射线能谱和射线质的改变考虑不够全面,造成计算结果和实际测量值之间有时会存在较大误差。本文通过对多个由测量数据反卷积得到的笔射束核做进一步处理,根据射野大小和计算平面深度做成阵列。在计算过程中,按照目标射野的照射条件,拟合出近似的笔射束核,在保证计算速度的同时,最大限度的保证了模型的计算精确度。实验证实,基于笔射束核阵列的剂量计算能够取得比较满意的结果,是一种可行的解决方案。
在论文的最后,我们对研究工作中的遗留问题进行了讨论,并对今后的工作进行了展望。
Together with surgery and chemotherapy, radiotherapy plays an important role in oncology, both in the definitive and palliative aspects of treatment. It treats with about 60%-70% patients suffering from cancer and cures 40% of them. The purpose of radiotherapy is to achieve the best treatment, which is dependent on maximizeing the dose to the tumor while minimizeing the dose to surrounding normal tissue and organs at risk from radiation.Radiotherapy is one of the most important methods for tumor treatment and engages computer radiotherapy treatment planning system as its key tool.In this thesis, we focused on one of the key problems:dose calculation, which is crucial to develop a radiotherapy treatment planning system.
With the development of oncology, the improvement of precise technology on radiotherapy has never stopped. In 1949, Swedish scientists Leksell proposed the concept of radiosurgery using stereotactic techniques, delivery high dose beam ray to the target and destroyed tumor in only one time, then the technology was named stereotactic radiosurgery (SRS), and it had be used in clinical application in 1955. In 60s of last century, Japanese scientists via a motor-driven collimator control the shape of beam, making the beam cross-section coincided with target projection in BEV derection. The early 70s, American scientists began to realize the importance of intensity modulated radiotherapy (IMRT), and studied on it. Late 80s early 90s, the United States, Germany and other countries had developed a commercial X-knife; Sweden had developed third generation of y-knife system; In the same period "Cyber Knife"came out. In 1995,the conception was proposed which use dynamic MLC in IMRT by Spirou et al and started to apply in prostate cancer treatment. In the new century, with the development of radiation physics, radiation biology, clinical oncology, medical imaging and other related disciplines, radiation therapy technology has undergone tremendous changed. Precision radiation therapy is to "precise positioning, accurate planning and precise treatment " for the characteristics of the new radiotherapy technology collectively. Precise radiation therapy in cancer treatment technology enables high accuracy, high dose, high efficacy and low damage the modern radiation patterns possible.
Precise radiotherapy compared with conventional radiotherapy technology has great progress, include:
(1) Commonly precise radiotherapy used CT, MRI, PET and other advanced imaging technology for determining the target and vital organs, and conventional radiation therapy commonly used X-ray simulator images;
(2) Modern radiotherapy always together with TPS.It is necessary to reconstruct and display image datas in three-dimensional is dependent by designed treatment plan.and 3-d display about dose distribution, and conventional radiation therapy techniques are only for 2-D treatment plan design;
(3) Modern radiotherapy commonly adopt mask, vacuum pads, head ring to combine with 3-D coordinate system, in order to repeat target position accurately. Generally conventional radiotherapy techniques such as surface lesions compared outlined rough positioning methods.
(4) Morden radiotherapy uses multiple non-coplanar radiation field or multiple non-coplanar arc radiation radiation, rather than conventional radiotherapy techniques use single or two fied irradiation, three field irradiation;
(5) Precise radiotherapy enables high dose area and target shape more consistent, and to ensure that inside target and the surface dose equal everywhere. That is to achieve the appropriate dosage conformal, conventional radiotherapy can not be achieved.
Dose calculation is the criterion to evaluate the merits of the treatment planning, optimization and inverse treatment planning is also based on a treatment plan design. treatment planning system is developing, while the rapid development of its core content of the dose calculation models and methods are constantly evolving. The development mainly from the dose calculation model,which improve from correction-based to model-based dose calculation to changes in the pattern and the mutual penetration between them. Correction-based dose calculation model which is based on field data under the conditions of the standard measurement of depth dose, off-axis ratio, scatter factor, appended to the necessary amendments and combined with the physical conditions to reflect the actual radiation dose distribution in radiotherapy..As computer storage capacity and computing speed improving, more accurate dose calculation methods continue to be used in treatment planning system. Such methods are generally focus on dose calculation model,which take into account the interaction in the human body entirely, in order to make a perfect agreement between the dose distribution based on theoretical and experimental measurements.In this paper, two categories of methods described in detail, focusing particularly on several model-based dose calculation methods analysis and research. By following,:
Dose distribution in the phantom is separated into the primary and scattered contribution of radiation. Such method include Clarkson method, differential scattering air ratio method, Day's law.
Monte Carlo is to use the principles of statistical methods to simulate a large number of single-energy photons and interact in the transport process in the phantom. Photons incident a medium, through the interaction of particle in the medium, and transfer a part or entire of ionizing radiation energy. Advantage of the method it applies mainly in the phantom arbitrary geometry and a variety of secondary particles on.
Another accurate model of dose calculation is convolution. In this model, the radiation dose calculated by energy fluence convolute with a point spread function or pencil beam kernel. Convolution method for distribution of any luminous radiation field, so this method is particularly suitable for irregular field calculations. In addition, because the convolution can be done by fast Fourier transform, convolution operation which makes the speed has been significantly enhanced.
Through the several model-based dose calculation methods were analyzed. Convolution is selected for TPS, and the principles of the model is researched deeply. The method has been improved on the basis and evolved accuracy of model. Convolution model as one of the mainstream models currently applied to TPS, When assume the kernel is an invariate with depth change,the model is simplified a plane fluence distribution convolute wth a pencil beam kernel. Dose distribution in the plane which plumb to incident beam at a certain depth calculated by convolution. For pencil beam kernel used in model, according to Fermi theory, in the case of small angle scattering, can be expressed as two independent one-dimensional product in perpendicular direction
One-dimensional pencil beam kernel can be derived from measured broad beam profile.Then it can be used to reconstruct beam profiles of various field sizes for photons.For the studies we have done so far,the agreement between measured profiles and calculated profiles is excellent.This method has the advantage that the kernel thus obtained include all physical processes as well as other perturbations such as scattering from various components of the treatment machine head and variation of energy spectrum with depth and are therefore more realistic. Futhermore, the require data, namely, central axis depth dose and beam profiles,are routinely measured data and readily available. But there are still some problems about this method. Use a single pencil beam kernel for reconstruct dose distribution of field profiles,it is will fail because the affect of field size and energy spectrum.,lead to deviation of measurement and calculaion. Based on the number of kernesl from the measurement data,a kernel array according to field size and depth is builded up. In the process of calculation, accordance with the target of radiant conditions fit an approximate kermel, while ensuring the calculation speed, the maximum guarantee the accuracy of the model calculations. The experiment confirmed that satisfactory results can be achieved is a feasible solution.
At the end of this thesis, we also discuss some problems which haven't been completed. In addition, we prospect some work which we'll do in the future.
伴随着放射肿瘤学的发展,关于精确放射治疗技术的研究从未停止过。1949年,瑞典科学家Leksell首先提出放射外科学的构想,利用立体定向技术,使用大剂量的射束一次性摧毁需治疗的病灶,此后这项治疗方法被命名为立体定向放射外科(SRS),并在1955年应用于临床。上世纪60年代初期,日本科学家用一种电机驱动光栅控制放射线束的形状,使得射束的截面正好吻合病灶的投影。70年代初期,美国科学家开始认识到调强放疗(IMRT)的重要性并对其进行了初步研究。80年代末90年代初,美、德等国家相继开发出了商用的X刀；瑞典开发了第三代γ刀系统；同时期带有机械手的"Cyber刀”出现。1995年Spirou等人提出了使用动态多叶光栅来实现调强放疗,并用其治疗前列腺癌。进入新世纪以来,随着放射物理学、放射生物学、临床肿瘤学和医学影像学等相关学科的发展,放射治疗技术领域发生了巨大变革。精确放射治疗技术就是以“精确定位、精确计划、精确治疗”为特征的新的放射治疗技术的统称。精确放射治疗技术使得在肿瘤治疗上实现高精度、高剂量、高疗效和低损伤(三高一低)的现代放疗模式成为可能。
精确放射治疗技术较之常规放射治疗技术有很大的进步,主耍表现在：
(1)精确放射治疗技术一般采用CT、MRI、PET等先进的影像技术作为确定靶区与重要器官的依据,而常规放射治疗技术一般采用X光模拟定位图象；
(2)精确放射治疗技术一般具有三维治疗计划系统(TPS),需对影像数据进行三维重建和三维显示,并在此基础上进行三维计划设计,显示三维剂量分布,而常规放射治疗技术一般只能进行二维治疗计划设计；
(3)精确放射治疗技术一般采用面膜、真空负压垫、头环等固定装置结合三维坐标系统,可对病灶进行单次或多次重复的准确定位,而常规放射治疗技术一般采用体表勾画病灶等比较粗糙的定位方法；
(4)精确放射治疗技术一般采用多个非共面照射野或多个非共面照射弧的照射方式,而常规放射治疗技术一般采用单野照射或两野、三野照射；
(5)精确放射治疗技术可使高剂量区域与靶区在三维形状上更加一致,并保证靶区内及表面的剂量处处相等。也即实现剂量上的适形,常规照射治疗技术则无法达到。
剂量计算的结果作为评价治疗计划的依据,同时也是治疗计划优化与逆向治疗计划设计的基础。在计算机放射治疗计划系统迅速发展的同时,其核心内容之一的剂量计算模型和方法也在不断地发展。这种发展主要表现为从基于修正的剂量计算模式向基于模型的剂量计算模式的转变以及二者的相互渗透。基于修正的剂量计算模式是以参考条件下标准野的实验测量深度剂量、离轴比、散射因子等为基础,加之必要修正后得到实际放射物理条件和放射治疗对象剂量分布的剂量计算方法。随着计算机存储容量和运算速度的不断提高,其它更高精度的剂量计算方法不断地被用于计算机放射治疗计划系统中。这类计算方法一般以建立新的剂量计算模型为着眼点,全局考虑放射线与人体的相互作用,从而使基于理论计算的剂量分布与基于实验测量的剂量分布更吻合。文中对这两大类方法进行了较为详细的介绍,特别是集中对几种基于模型的剂量计算方法进行了详细的分析和研究。其中包括：将模体中任意一点的剂量分成原射线剂量和散射线剂量两部分的剂量计算方法被称作原射线剂量和散射线剂量分离法。此类计算包括了Clarkson法、微分散射空气比法、Day氏法等。
相比于上述几种方法,学术界普遍认为模特卡罗法为目前剂量计算的金标准。该方法进行剂量计算的原理为用统计学方法来模拟大量单能光子在输运过程中与物质的作用。光子进人某种介质(如人体组织)后,通过与介质中原子的相互作用,而传递电离辐射的部分或全部能量。该方法的先进之处主要表现在它能适用于任意几何形状的体模和各种次级粒子(如光子辐射产生的电子)上。
另一个比较精确的剂量计算模型是卷积法。在这种模型中,剂量计算是通过将放射线的光通分布与一个点扩展函数或卷积核相卷积来实现的。卷积法适用于任意的射野光通分布,所以这种方法特别适宜于不规则野的计算。另外,因为卷积可通过快速傅立叶变换来完成,这使卷积运算速度得到了显著地提高。
通过对上述几种基于模型的剂量计算方法进行了分析,选定采用卷积模型完成剂量计算,并且对该模型的原理进行了深入的研究,在该方法已有的理论基础上进行了改进,提高了模型的计算精确度。
卷积模型作为当前应用于放射治疗计划系统中的主流计算模型之一。当暂不考虑卷积核随深度的变化情况时,简化为平面内的注量分布与笔射束核的卷积。通过计算可以得到某一深度处于射束方向垂直的平面内的剂量分布。对于卷积运算中所需要用到的笔射束核,根据Fermi理论,在小角度散射的情况下,可以表示为两个相互独立的正交方向上的一维笔射束核的乘积形式。
一维笔射束核可以通过实际测量的宽平行束的剂量分布数据求解得到。再利用笔射束核重建各种形状射野的剂量分布,具有一定的计算精度的优势,减少了额外工作量。该方法的优点是：测量数据综合了照射过程的各种散射因素,模型不存在理想化和简化的问题。利用笔射束核重建测量野自身或受照条件与之相近的射野的剂量分布效果较好。但采用单一笔射束核重建任意射野剂量分布仍存在一定问题。
进入模体内的光子与介质相互作用产生次级电子将能量传递给介质,在建成区附近达到电子平衡。卷积模型中假设笔射束核具有空间不变性的这一前提,对直线加速器而言在达到电子平衡的区域外难以得到保证。在直线加速器的剂量分布中,散射线的剂量贡献占有极其重要的一部分。均匀模体中散射线剂量受射野大小和深度,以及同一方向上准直器的近端和远端边界的影响,其变化较为复杂。在较大深度处,模体内散射占主要；较浅深度处,受准直器散射影响较多。由于各种干扰因素的存在,造成射线能谱发生改变,使实际笔射束核的剂量分布并非是一个空间不变量,受到笔射束核为空间不变量这一假设的限制,对其进行修正,会减弱卷积模型可以通过快速傅里叶变换完成的这一优点。利用单一的笔射束核对任意射野的剂量分布进行重建,因为对射线能谱和射线质的改变考虑不够全面,造成计算结果和实际测量值之间有时会存在较大误差。本文通过对多个由测量数据反卷积得到的笔射束核做进一步处理,根据射野大小和计算平面深度做成阵列。在计算过程中,按照目标射野的照射条件,拟合出近似的笔射束核,在保证计算速度的同时,最大限度的保证了模型的计算精确度。实验证实,基于笔射束核阵列的剂量计算能够取得比较满意的结果,是一种可行的解决方案。
在论文的最后,我们对研究工作中的遗留问题进行了讨论,并对今后的工作进行了展望。
Together with surgery and chemotherapy, radiotherapy plays an important role in oncology, both in the definitive and palliative aspects of treatment. It treats with about 60%-70% patients suffering from cancer and cures 40% of them. The purpose of radiotherapy is to achieve the best treatment, which is dependent on maximizeing the dose to the tumor while minimizeing the dose to surrounding normal tissue and organs at risk from radiation.Radiotherapy is one of the most important methods for tumor treatment and engages computer radiotherapy treatment planning system as its key tool.In this thesis, we focused on one of the key problems:dose calculation, which is crucial to develop a radiotherapy treatment planning system.
With the development of oncology, the improvement of precise technology on radiotherapy has never stopped. In 1949, Swedish scientists Leksell proposed the concept of radiosurgery using stereotactic techniques, delivery high dose beam ray to the target and destroyed tumor in only one time, then the technology was named stereotactic radiosurgery (SRS), and it had be used in clinical application in 1955. In 60s of last century, Japanese scientists via a motor-driven collimator control the shape of beam, making the beam cross-section coincided with target projection in BEV derection. The early 70s, American scientists began to realize the importance of intensity modulated radiotherapy (IMRT), and studied on it. Late 80s early 90s, the United States, Germany and other countries had developed a commercial X-knife; Sweden had developed third generation of y-knife system; In the same period "Cyber Knife"came out. In 1995,the conception was proposed which use dynamic MLC in IMRT by Spirou et al and started to apply in prostate cancer treatment. In the new century, with the development of radiation physics, radiation biology, clinical oncology, medical imaging and other related disciplines, radiation therapy technology has undergone tremendous changed. Precision radiation therapy is to "precise positioning, accurate planning and precise treatment " for the characteristics of the new radiotherapy technology collectively. Precise radiation therapy in cancer treatment technology enables high accuracy, high dose, high efficacy and low damage the modern radiation patterns possible.
Precise radiotherapy compared with conventional radiotherapy technology has great progress, include:
(1) Commonly precise radiotherapy used CT, MRI, PET and other advanced imaging technology for determining the target and vital organs, and conventional radiation therapy commonly used X-ray simulator images;
(2) Modern radiotherapy always together with TPS.It is necessary to reconstruct and display image datas in three-dimensional is dependent by designed treatment plan.and 3-d display about dose distribution, and conventional radiation therapy techniques are only for 2-D treatment plan design;
(3) Modern radiotherapy commonly adopt mask, vacuum pads, head ring to combine with 3-D coordinate system, in order to repeat target position accurately. Generally conventional radiotherapy techniques such as surface lesions compared outlined rough positioning methods.
(4) Morden radiotherapy uses multiple non-coplanar radiation field or multiple non-coplanar arc radiation radiation, rather than conventional radiotherapy techniques use single or two fied irradiation, three field irradiation;
(5) Precise radiotherapy enables high dose area and target shape more consistent, and to ensure that inside target and the surface dose equal everywhere. That is to achieve the appropriate dosage conformal, conventional radiotherapy can not be achieved.
Dose calculation is the criterion to evaluate the merits of the treatment planning, optimization and inverse treatment planning is also based on a treatment plan design. treatment planning system is developing, while the rapid development of its core content of the dose calculation models and methods are constantly evolving. The development mainly from the dose calculation model,which improve from correction-based to model-based dose calculation to changes in the pattern and the mutual penetration between them. Correction-based dose calculation model which is based on field data under the conditions of the standard measurement of depth dose, off-axis ratio, scatter factor, appended to the necessary amendments and combined with the physical conditions to reflect the actual radiation dose distribution in radiotherapy..As computer storage capacity and computing speed improving, more accurate dose calculation methods continue to be used in treatment planning system. Such methods are generally focus on dose calculation model,which take into account the interaction in the human body entirely, in order to make a perfect agreement between the dose distribution based on theoretical and experimental measurements.In this paper, two categories of methods described in detail, focusing particularly on several model-based dose calculation methods analysis and research. By following,:
Dose distribution in the phantom is separated into the primary and scattered contribution of radiation. Such method include Clarkson method, differential scattering air ratio method, Day's law.
Monte Carlo is to use the principles of statistical methods to simulate a large number of single-energy photons and interact in the transport process in the phantom. Photons incident a medium, through the interaction of particle in the medium, and transfer a part or entire of ionizing radiation energy. Advantage of the method it applies mainly in the phantom arbitrary geometry and a variety of secondary particles on.
Another accurate model of dose calculation is convolution. In this model, the radiation dose calculated by energy fluence convolute with a point spread function or pencil beam kernel. Convolution method for distribution of any luminous radiation field, so this method is particularly suitable for irregular field calculations. In addition, because the convolution can be done by fast Fourier transform, convolution operation which makes the speed has been significantly enhanced.
Through the several model-based dose calculation methods were analyzed. Convolution is selected for TPS, and the principles of the model is researched deeply. The method has been improved on the basis and evolved accuracy of model. Convolution model as one of the mainstream models currently applied to TPS, When assume the kernel is an invariate with depth change,the model is simplified a plane fluence distribution convolute wth a pencil beam kernel. Dose distribution in the plane which plumb to incident beam at a certain depth calculated by convolution. For pencil beam kernel used in model, according to Fermi theory, in the case of small angle scattering, can be expressed as two independent one-dimensional product in perpendicular direction
One-dimensional pencil beam kernel can be derived from measured broad beam profile.Then it can be used to reconstruct beam profiles of various field sizes for photons.For the studies we have done so far,the agreement between measured profiles and calculated profiles is excellent.This method has the advantage that the kernel thus obtained include all physical processes as well as other perturbations such as scattering from various components of the treatment machine head and variation of energy spectrum with depth and are therefore more realistic. Futhermore, the require data, namely, central axis depth dose and beam profiles,are routinely measured data and readily available. But there are still some problems about this method. Use a single pencil beam kernel for reconstruct dose distribution of field profiles,it is will fail because the affect of field size and energy spectrum.,lead to deviation of measurement and calculaion. Based on the number of kernesl from the measurement data,a kernel array according to field size and depth is builded up. In the process of calculation, accordance with the target of radiant conditions fit an approximate kermel, while ensuring the calculation speed, the maximum guarantee the accuracy of the model calculations. The experiment confirmed that satisfactory results can be achieved is a feasible solution.
At the end of this thesis, we also discuss some problems which haven't been completed. In addition, we prospect some work which we'll do in the future.
引文
[1]勾成俊.基于混合笔束模型的三维电子剂量计算研究[D]四川大学博士论文,2002
[2]Ira J. Kalet. Multiview three-dimensional treatment planning[J]. Front. Radiat. Ther. Oncol., 1987, (21):33-43.
[3]Dutreix A, van der Schueren E, Derreumaux S,et al.Preliminary results of a quality assurance network for radiotherapy centres in Europe[J]. Radiother Oncol.1993 Nov;29(2):97-101.
[4]Anders Atmesj, Mafia Mania Aspeadakis. Dose calculations for external photon beams in radiotherapy[J]. Phy. Med. Biol.1999,44(11):99-155.
[5]翁学军.放射治疗剂量计算的模特卡罗方法研究[D].东南大学博士论文,2003
[6]Boyer A L and Schultheiss T. Effects of dosimetric and clinical uncertainty on complication-free local tumor control[J]. Radiotherapy Oncology.1988.11(1):65-71.
[7]ICRU. Report 24. Determination of absorbed dose in a patient irradiated by beams of X or Gamma rays in radiotherapy procedures[R].1976.
[8]胡逸民等,肿瘤放射物理学.[M]原子能出版社(北京),1999年9月。
[9]Thomas SJ, Eaton DJ, Tudor GS, Twyman NI.. Equivalent squares for small field dosimetry [J].Br. J. Radiol.,2008,81(971):897-901.
[10]Day MJ.A note on the calculation of dose on X-ray field[J].Br J Radiol 1950,23(270):368-369.
[11]M. J. Day, E. G. A. Aird. The equivalent field method for dose determination in rectangular fields[J]. Br. J. Radiol. Suppl.,1983,17(Suppl):105-114.
[12]Van De Geijn J.The computation of two and three dimensional dose distributions in Cobalt-60 teletherapy[J].Br J Radiol 1965.38.(5):369-377.
[13]Radhe Mohan PH.D, Glenn Barest B.S et al. A comprehensive three-dimensional radiation treatment planning system[J]. Int J Radiat Oncol Biol Phys.1988,15(2):481-495.
[14]Chui CS, Mohan R. Off-center ratios for three-dimensional dose calculations[J]. Med. Phys. 1986,13(3):409-412.
[15]Gunilla C.Bentel. Radiation Therapy Planning(first edition)[M].New York:Macmillan Publishing Company,1992.1-23
[16]殷蔚伯,谷铣之,肿瘤放射治疗学(第三版)[M].北京：中国协和医科大学出版社,2002：53-56
[17]Khan FM,Gerbi BJ,et al.Dosimetry of asymmetric X-ray collimators[J].Med Phys 1986,13(6):936-941.
[18]Leavitt DD,Martin M,et al.Dynamic wedge field technique through computer controlled collimator motion and dose delivery[J].Med Phys.1990,17(1):87-91.
[19]Milan J., Bentley R. E. The storage and manipulation of radiation-dose data in a small digital computer[J]. Br. J. Radiol.,1974,47(554):115-121.
[20]Meredith W. J., Neary G. J. The production of isodose curves and calculation of energy absorption from standard depth dose data[J]. Br. J. Radiol.,1944, (17):126-130.
[21]Cunningham J. R. Scatter-air ratios[J], Phys. Med. Biol., 1972.17(1):42-51.
[22]Johns H. E., Cunningham J. R. The physics of radiology,4th Edition[M]. Springfield,1983
[23]Paul J. M., Koch R. F. Characteristics of mevatron 77 15-MV photon beam[J]. Med. Phys., 1983,10(2):237.
[24]Bruinvis IA, Mathol WA.Calculation of electron beam depth-dose curves and output factors for arbitrary field shapes [J]. Radiother Oncol.,1988,11(4):395-404.
[25]Day M. J. A note on the calculation of dose in fields of irregular shape[J]. Br. J. Radiol.,1941, (14):265-268.
[26]Haywood J. K., Bomford C. K. A less empirical method of representing megavoltage beams for use in rapid radiotherapy dose calculations[J]. Br. J. Radiol.,1979,52(621):709-718.
[27]Arthur Boyer, Ed Mok. A photon dose distribution model employing convolution calculations[J].Med. Phys.1985,12(2):169-177.
[28]Mohan R., Chui C. S. Use of fast Fourier transforms in calculating dose distributions for irregular shaped fields for three dimensional treatment planning[J]. Med. Phys.,1987,14(1): 70-77.
[29]Webb S. The absorbed dose in the vicinity of an interface between two media irradiated by a Co-60 source[J]. Br. J. Radiol.,1979,52(624),962-967.
[30]Bourland JD, Chaney EL. A finite-size pencil beam model for photon dose calculations in three dimensions[J]. Med Phys.1992,19(6):1401-1412.
[31]Bortfeld T, Schlegel W, Rhein B. Decomposition of pencil beam kernels for fast dose calculations in three-dimensional treatment planning[J]. Med. Phys.1993;20(2):311-318.
[32]Mah E,Antolak J,Scrimger J.W.et al.Experimental evaluation of 2D and 3D electron pencil beam algrithm[J].Phys.Med.Biol.,1989,34(9):1179-1194
[33]Crister P.Ceberg,Benge E.Bjarngard.Experimential detemination of the dose kernel in high-energy X-ray beam[J].Med.Phys.1996,23(4):505-511.
[34]Lei D. Almon S. A pencil beam photon dose algorithm for stereotactic radiosurgey using a miniature multifleaf collimator[J]. Med. Phys.,1998,25(6):841-850.
[35]Bevington P. R. Data reduction and error analysis for the physics science[J]. McGraw-Hill, New York,1969.
[36]Storchi PR, van Battum LJ, Woudstra E. Calculation of a pencil beam kernel from measured photon beam data.[J] Phys Med Biol.1999,44(12):2917-2928.
[37]Chui C. S., Mohan R. Extraction of pencil beam kernels by the deconvolution method[J]. Med. Phys.,1988,15(2):138-144.
[38]Storchi P, Woudstra E. Calculation of the absorbed dose distribution due to irregularly shaped photon beams using pencil beam kernels derived form basic beam data[J]. Phys Med Biol.1996, 41(4):637-656.
[39]D. Jette, A. Pagnamenta, L. H. Lanzel, and M. Rozenfeld. The application of multiple scattering theory to therapeutic electron dosimetry[J]. Med. Phys.,1983,10(2):141-146.
[2]Ira J. Kalet. Multiview three-dimensional treatment planning[J]. Front. Radiat. Ther. Oncol., 1987, (21):33-43.
[3]Dutreix A, van der Schueren E, Derreumaux S,et al.Preliminary results of a quality assurance network for radiotherapy centres in Europe[J]. Radiother Oncol.1993 Nov;29(2):97-101.
[4]Anders Atmesj, Mafia Mania Aspeadakis. Dose calculations for external photon beams in radiotherapy[J]. Phy. Med. Biol.1999,44(11):99-155.
[5]翁学军.放射治疗剂量计算的模特卡罗方法研究[D].东南大学博士论文,2003
[6]Boyer A L and Schultheiss T. Effects of dosimetric and clinical uncertainty on complication-free local tumor control[J]. Radiotherapy Oncology.1988.11(1):65-71.
[7]ICRU. Report 24. Determination of absorbed dose in a patient irradiated by beams of X or Gamma rays in radiotherapy procedures[R].1976.
[8]胡逸民等,肿瘤放射物理学.[M]原子能出版社(北京),1999年9月。
[9]Thomas SJ, Eaton DJ, Tudor GS, Twyman NI.. Equivalent squares for small field dosimetry [J].Br. J. Radiol.,2008,81(971):897-901.
[10]Day MJ.A note on the calculation of dose on X-ray field[J].Br J Radiol 1950,23(270):368-369.
[11]M. J. Day, E. G. A. Aird. The equivalent field method for dose determination in rectangular fields[J]. Br. J. Radiol. Suppl.,1983,17(Suppl):105-114.
[12]Van De Geijn J.The computation of two and three dimensional dose distributions in Cobalt-60 teletherapy[J].Br J Radiol 1965.38.(5):369-377.
[13]Radhe Mohan PH.D, Glenn Barest B.S et al. A comprehensive three-dimensional radiation treatment planning system[J]. Int J Radiat Oncol Biol Phys.1988,15(2):481-495.
[14]Chui CS, Mohan R. Off-center ratios for three-dimensional dose calculations[J]. Med. Phys. 1986,13(3):409-412.
[15]Gunilla C.Bentel. Radiation Therapy Planning(first edition)[M].New York:Macmillan Publishing Company,1992.1-23
[16]殷蔚伯,谷铣之,肿瘤放射治疗学(第三版)[M].北京：中国协和医科大学出版社,2002：53-56
[17]Khan FM,Gerbi BJ,et al.Dosimetry of asymmetric X-ray collimators[J].Med Phys 1986,13(6):936-941.
[18]Leavitt DD,Martin M,et al.Dynamic wedge field technique through computer controlled collimator motion and dose delivery[J].Med Phys.1990,17(1):87-91.
[19]Milan J., Bentley R. E. The storage and manipulation of radiation-dose data in a small digital computer[J]. Br. J. Radiol.,1974,47(554):115-121.
[20]Meredith W. J., Neary G. J. The production of isodose curves and calculation of energy absorption from standard depth dose data[J]. Br. J. Radiol.,1944, (17):126-130.
[21]Cunningham J. R. Scatter-air ratios[J], Phys. Med. Biol., 1972.17(1):42-51.
[22]Johns H. E., Cunningham J. R. The physics of radiology,4th Edition[M]. Springfield,1983
[23]Paul J. M., Koch R. F. Characteristics of mevatron 77 15-MV photon beam[J]. Med. Phys., 1983,10(2):237.
[24]Bruinvis IA, Mathol WA.Calculation of electron beam depth-dose curves and output factors for arbitrary field shapes [J]. Radiother Oncol.,1988,11(4):395-404.
[25]Day M. J. A note on the calculation of dose in fields of irregular shape[J]. Br. J. Radiol.,1941, (14):265-268.
[26]Haywood J. K., Bomford C. K. A less empirical method of representing megavoltage beams for use in rapid radiotherapy dose calculations[J]. Br. J. Radiol.,1979,52(621):709-718.
[27]Arthur Boyer, Ed Mok. A photon dose distribution model employing convolution calculations[J].Med. Phys.1985,12(2):169-177.
[28]Mohan R., Chui C. S. Use of fast Fourier transforms in calculating dose distributions for irregular shaped fields for three dimensional treatment planning[J]. Med. Phys.,1987,14(1): 70-77.
[29]Webb S. The absorbed dose in the vicinity of an interface between two media irradiated by a Co-60 source[J]. Br. J. Radiol.,1979,52(624),962-967.
[30]Bourland JD, Chaney EL. A finite-size pencil beam model for photon dose calculations in three dimensions[J]. Med Phys.1992,19(6):1401-1412.
[31]Bortfeld T, Schlegel W, Rhein B. Decomposition of pencil beam kernels for fast dose calculations in three-dimensional treatment planning[J]. Med. Phys.1993;20(2):311-318.
[32]Mah E,Antolak J,Scrimger J.W.et al.Experimental evaluation of 2D and 3D electron pencil beam algrithm[J].Phys.Med.Biol.,1989,34(9):1179-1194
[33]Crister P.Ceberg,Benge E.Bjarngard.Experimential detemination of the dose kernel in high-energy X-ray beam[J].Med.Phys.1996,23(4):505-511.
[34]Lei D. Almon S. A pencil beam photon dose algorithm for stereotactic radiosurgey using a miniature multifleaf collimator[J]. Med. Phys.,1998,25(6):841-850.
[35]Bevington P. R. Data reduction and error analysis for the physics science[J]. McGraw-Hill, New York,1969.
[36]Storchi PR, van Battum LJ, Woudstra E. Calculation of a pencil beam kernel from measured photon beam data.[J] Phys Med Biol.1999,44(12):2917-2928.
[37]Chui C. S., Mohan R. Extraction of pencil beam kernels by the deconvolution method[J]. Med. Phys.,1988,15(2):138-144.
[38]Storchi P, Woudstra E. Calculation of the absorbed dose distribution due to irregularly shaped photon beams using pencil beam kernels derived form basic beam data[J]. Phys Med Biol.1996, 41(4):637-656.
[39]D. Jette, A. Pagnamenta, L. H. Lanzel, and M. Rozenfeld. The application of multiple scattering theory to therapeutic electron dosimetry[J]. Med. Phys.,1983,10(2):141-146.