Keen Medical Physics Services, Tokyo, Japan.
Med Phys. 2011 Oct;38(10):5818-29. doi: 10.1118/1.3641829.
The purpose of this study is to develop a new calculation algorithm that is satisfactory in terms of the requirements for both accuracy and calculation time for a simulation of imaging of the proton-irradiated volume in a patient body in clinical proton therapy.
The activity pencil beam algorithm (APB algorithm), which is a new technique to apply the pencil beam algorithm generally used for proton dose calculations in proton therapy to the calculation of activity distributions, was developed as a calculation algorithm of the activity distributions formed by positron emitter nuclei generated from target nuclear fragment reactions. In the APB algorithm, activity distributions are calculated using an activity pencil beam kernel. In addition, the activity pencil beam kernel is constructed using measured activity distributions in the depth direction and calculations in the lateral direction. (12)C, (16)O, and (40)Ca nuclei were determined as the major target nuclei that constitute a human body that are of relevance for calculation of activity distributions. In this study, "virtual positron emitter nuclei" was defined as the integral yield of various positron emitter nuclei generated from each target nucleus by target nuclear fragment reactions with irradiated proton beam. Compounds, namely, polyethylene, water (including some gelatin) and calcium oxide, which contain plenty of the target nuclei, were irradiated using a proton beam. In addition, depth activity distributions of virtual positron emitter nuclei generated in each compound from target nuclear fragment reactions were measured using a beam ON-LINE PET system mounted a rotating gantry port (BOLPs-RGp). The measured activity distributions depend on depth or, in other words, energy. The irradiated proton beam energies were 138, 179, and 223 MeV, and measurement time was about 5 h until the measured activity reached the background level. Furthermore, the activity pencil beam data were made using the activity pencil beam kernel, which was composed of the measured depth data and the lateral data including multiple Coulomb scattering approximated by the Gaussian function, and were used for calculating activity distributions.
The data of measured depth activity distributions for every target nucleus by proton beam energy were obtained using BOLPs-RGp. The form of the depth activity distribution was verified, and the data were made in consideration of the time-dependent change of the form. Time dependence of an activity distribution form could be represented by two half-lives. Gaussian form of the lateral distribution of the activity pencil beam kernel was decided by the effect of multiple Coulomb scattering. Thus, the data of activity pencil beam involving time dependence could be obtained in this study.
The simulation of imaging of the proton-irradiated volume in a patient body using target nuclear fragment reactions was feasible with the developed APB algorithm taking time dependence into account. With the use of the APB algorithm, it was suggested that a system of simulation of activity distributions that has levels of both accuracy and calculation time appropriate for clinical use can be constructed.
本研究旨在开发一种新的计算算法，该算法在准确性和计算时间方面都能满足临床质子治疗中模拟质子辐照患者体内体积成像的要求。
作为一种将质子治疗中通常用于质子剂量计算的铅笔束算法（APB 算法）应用于放射性核素分布计算的新技术，我们开发了一种用于计算由靶核碎片反应产生的正电子发射核形成的放射性核素分布的计算算法。在 APB 算法中，使用放射性核素铅笔束核来计算放射性核素分布。此外，放射性核素铅笔束核是通过在深度方向上测量放射性核素分布和在横向方向上的计算来构建的。（12）C、（16）O 和（40）Ca 核被确定为与计算放射性核素分布相关的构成人体的主要靶核。在这项研究中，“虚拟正电子发射核”被定义为通过与辐照质子束的靶核碎片反应，由每个靶核产生的各种正电子发射核的积分产额。使用质子束辐照含有大量靶核的化合物，即聚乙烯、水（包括一些明胶）和氧化钙。此外，使用安装在旋转机架端口（BOLPs-RGp）上的在线 PET 系统测量从靶核碎片反应中在每个化合物中产生的虚拟正电子发射核的深度放射性核素分布。所测量的放射性核素分布取决于深度，换句话说，取决于能量。辐照质子束能量分别为 138、179 和 223 MeV，测量时间约为 5 小时，直到测量到的放射性核素达到背景水平。此外，使用由测量的深度数据和包括用高斯函数近似的多次库仑散射的横向数据组成的放射性核素铅笔束核来制作放射性核素铅笔束数据，并用于计算放射性核素分布。
使用 BOLPs-RGp 获得了每个靶核的质子束能量的深度放射性核素分布数据。验证了深度放射性核素分布的形式，并考虑了形式的时变。放射性核素分布形式的时间依赖性可以用两个半衰期来表示。活性铅笔束核的横向分布的高斯形式是由多次库仑散射的影响决定的。因此，在这项研究中可以获得涉及时间依赖性的活性铅笔束数据。
考虑到时间依赖性，使用开发的 APB 算法可以对基于靶核碎片反应的患者体内质子辐照体积成像进行模拟。使用 APB 算法，建议可以构建一种具有适当准确性和计算时间水平的适用于临床应用的活性分布模拟系统。
