基于蒙特卡洛模拟的肺癌患者体内光子能谱和散射次级电子能谱研究

Photon energy spectrum and scattering secondary electron energy spectrum of lung cancer patients based on Monte Carlo simulation

目的放射治疗是恶性肿瘤的重要治疗手段,且治疗的精准度越来越高。针对患者的个体化生物特征,设计更精准的个体化放疗方案已引起研究人员的重视。该文研究放疗中高能光子束进入患者肿瘤及正常组织后光子能谱和散射次级电子能谱规律,为放射生物剂量个体化计算奠定理论基础。方法将患者的CT图像序列经重采样,体元尺寸为2. 54 mm×2. 54 mm×2. 5 mm,并将体元的CT值转换成材料的元素组成密度。利用蒙特卡洛模拟方法计算各受照射体元的光子能谱和散射次级电子能谱,并分析能谱规律。结果能谱在250～300 ke V达到峰值,平均能量范围为900～1150 ke V,不同入射深度处的光子能谱具有较好的一致性。散射次级电子能谱可分为两种类型:①高密度区域(肿瘤组织),散射次级电子平均能量较高(平均能量约为909. 58 ke V)且能谱存在展宽;②低密度区域,散射次级电子平均能量较低(平均能量为536 ke V)且能谱无展宽。结论该研究给出了肺癌患者体内光子能谱和散射次级电子能谱的具体分布规律和表达,为在细胞及DNA层面研究放射生物剂量计算模型奠定了基础。

Objective To lay the theoretical foundation for the individualized radiation biological dose calculation by studing the energy spectra of incident photons and scattered secondary electrons in photon beam radiotherapy. Methods The patients’ CT images were resampled,voxel size of 2. 54 mm × 2. 54 mm × 2. 5 mm was obtained and the CT values of voxels were converted into the density of the material. Then,we calculated the energy spectra of photons and scattered secondary electrons using the Monte Carlo simulation method. Based on the results of calculation,we analyzed the spectral features of the photons and electrons. Results The photon energy spectrum’ s peak energy was at about 250 to 300 ke V,and the average energy ranged from about 900 to 1150 ke V. The photon energy spectra had little difference at different depth. There were two types of scattered electron energy spectra: ①in high density areas( tumor),the average energy of scattered secondary electrons was higher( average energy of 909. 58 ke V) and the spectrum was broadening;② in low density areas,the average energy of scattered secondary electrons was lower( average energy of 536 ke V) and the energy spectrum was not broadening. Conclusion The specific distribution features and expression of photon energy spectra and scattered secondary electron energy spectra in lung cancer patients are obtained,which has laid a foundation for studying the dose calculation model of radiobiology at the level of cells and DNA.