基于SRIM分析碳离子治疗中Bragg峰的分布特征

SRIM-based analysis of distribution features of Bragg peak of carbon ion radiotherapy

目的基于SRIM软件对碳离子束在材料中布拉格(Bragg)峰的分布特征进行分析,并探索利用CT值计算碳离子束入射能量。方法通过SRIM软件,研究能量范围在100~300 MeV/u的碳离子束入射到不同等效材料中的输运情况,分析碳离子入射能量、等效材料及厚度等对其Bragg峰深度的影响。采用Origin 2017进行数据拟合,分析CT值与Bragg峰深度水等效比间的函数关系。结果随着入射碳离子束能量增加,发现碳离子束在等效材料中的Bragg峰深度与水中Bragg峰深度的比值近乎为常数。通过CT值与Bragg峰深度水等效比D_(i)间的函数关系,能够将特定能量碳离子束在等效材料中的Bragg峰深度换算为水中等效Bragg峰深度。结论利用人体组织不同体积单元的Bragg峰深度水等效比D_(i)和CT值,能够精确算出Bragg峰落在肿瘤部位所需的水中等效Bragg峰深度,据此可反推出碳离子束所需的入射能量。

Objective To analyze the distribution features of the Bragg peak of carbon ion beams in materials using SRIM software,and to explore the use of computed tomography(CT)number to calculate the incident energy of carbon ion beams.Methods SRIM software was used to study the travel of carbon ion beams(100 to 300 MeV/u)in different equivalent materials,and analyze the effects of the incident energy of carbon ion beams and the type and thickness of equivalent materials on the depth of the Bragg peak of carbon ion beams.Origin 2017 was used to analyze the functional relationship between CT number and water-equivalent Bragg peak depth ratio(D_(i))through data fitting.Results The ratios of the Bragg peak depths in equivalent materials to that in water almost stayed constant with the increase in the incident energy of carbon ion beams.Through the functional relation between CT number and D_(i),the Bragg peak depth of a carbon ion beam of a given energy in an equivalent material could be converted to the equivalent Bragg peak depth in water.Conclusion With the water-equivalent Bragg peak depth ratio D_(i)and CT number of different volume units of human tissues,the equivalent Bragg peak depth in water required for the Bragg peak to fall in the tumor can be accurately calculated,which can be used to reversely infer the needed incident energy of carbon ion beams.