碳离子在不同材料叶片的多叶光栅上产生的次级粒子研究

在碳离子放射治疗中，碳离子束与治疗头设备和患者身体相互作用产生的次级粒子可以到达患者体内的许多区域，在产生的次级粒子中以中子和$\gamma $射线的产额为最大。在不影响束流配送功能的情况下，减少碳离子放疗中产生的次级中子和$\gamma $射线对于降低放疗后出现的正常组织并发症及二次肿瘤风险有着非常重要的意义。本文利用蒙特卡罗(Monte Carlo)方法模拟计算了被动式束流配送系统下，400 MeV/u碳离子束照射到由不同材料叶片构成的多叶光栅(MLC)形成典型的10 cm×10 cm方形射野时，在水模体中产生的次级中子和$\gamma $射线所沉积的剂量及空间分布等。模拟结果显示:碳离子束通过MLC形成射野后在水模体中产生的次级中子主要分布在水模体的入射端，次级$\gamma $射线较为均匀的分布在整个水模体内，且较多分布在具有展宽Bragg峰(SOBP)射野在水模体中贯穿时的坪区。对于MLC叶片材料的选择，则需根据实际情况对叶片厚度以及次级粒子当量剂量的要求来确定。本文通过模拟研究不同MLC叶片材料产生次级粒子的情况，为被动式束流配送系统中MLC叶片及其他元件的材料选择提供了科学依据。

Study on the Secondary Particles Produced in Different Material Leaves of Multi-leaf Collimator under Carbon Ion Irradiation

In carbon ion radiotherapy, secondary particles produced through the interaction of carbon ion beam with the treatment head devices and the patient's body can reach many areas in the patient's body. Among the secondary particles, the yield of neutrons and $\gamma $-rays is largest. Without affecting the beam delivery function, reducing secondary neutrons and $\gamma $-rays produced in carbon ion radiotherapy is very important to decrease the normal tissue complications and secondary cancer risk after radiotherapy. In the present work, the Monte Carlo method was used to calculate the dose and spatial distributions of secondary neutrons and $\gamma $-rays deposited in water when multi-leaf collimators (MLC) made from different material leaves were adopted to form a typical 10 cm×10 cm square irradiation field for 400 MeV/u carbon ion beam. The simulation results showed that the secondary neutrons produced were mainly distributed at the incident end in the water phantom when the carbon ion beam passed through the MLC, while the secondary $\gamma $-rays were uniformly distributed in the entire water phantom and more dose of the secondary $\gamma $-rays appeared in the plateau when the irradiation field with spread-out Bragg peak (SOBP) travelled in the water phantom. The selection of MLC leaves should be determined according to the actual conditions of MLC leaves thickness and secondary particle equivalent dose requirements. The simulation study presented in this paper provides scientific evidence for the material selection of MLC leaves and other components in carbon ion radiotherapy using passive beam delivery system.