中文摘要：

目的：计算放射治疗能区内的光子在类人体介质中的能谱分布。方法：使用特征线方法将介质中的光子分解为初级光子、一次散射光子和多次散射光子．相应地，光子的微分注量分布函数也被分成三个部分，分别是初级光子的微分注量分布函数、一次散射光子的微分注量分布函数和多次散射光子的微分注量分布函数．并由相应的玻尔兹曼方程求解各类光子的微分注量分布函数。总的光子微分注量分布函数是各部分光子的微分注量分布函数之和。将微分注量分布函数对角度进行积分后。便得到光子的能谱。将使用特征线方法计算的结果和蒙特卡罗方法的计算结果进行比较。结果：光子注量分布在大部分深度范围内。与蒙特卡罗方法的计算结果结果吻合得都很好，其误差小于2％。但是在接近水层的末端，特征线方法的计算结果略微高于蒙特卡罗方法的计算结果，其最大误差达到约7％。一次散射光子的能谱分布和蒙特卡罗方法的计算结果吻合得相当好。在表面处，能量主要集中在低能部分，在高于1MeV后，迅速下降到接近于0，能谱展宽较小。在中间部位，能谱在达到最大值后呈指数曲线形式下降。而水层在末端，能谱达到最大值后，基本保持一个常数。多次散射光子的能谱分布在大部分能量点上，特征线方法的计算结果和蒙特卡罗方法的计算结果吻合得很好。只是在极低能量处．二者的计算结果存在较大的差别。结论：在放射治疗能区．特征线方法和蒙特卡罗方法的结果吻合得相当好，特征线方法可以高效和准确地计算光子在介质中的输运问题。

英文摘要：

Purpose： To study energy spectrum of photon within radiotherapeutic energy in body-like material. Method： Dividing photons into three parts, named prime-scattered-photon, secondary-scattered-photon, and multi-scattered-photon in the characteristic line method, accordingly, the distribution functions of photons are called prime-scattered-photon, secondary-scattered-photon, and multi-scattered-photon differential fluence. We obtain each part of differential fluence by solving transport equations for photons. The total differential fluence is the sum of each part of photons.The energy spectrum is obtained by using the integral of differential fluence with opposit angle.The energy spectrtun and fluence distributions for 6MeV photon in water resulting from the characteristic line method compared with the results from Monte Carlo （MC） code. Result： The fluence distributions in most range of depth and the rusults from MC code well fit agreement.,the error less than 2%. But, they little more than the rusults from MC code at the end of water slab and the maximal error is 7%. The energy spectrums of secondary-photons are more closed with the rusults from MC code. At surface, the energy spectrums concentrate in low energy ,then drop down to 0 sharply when their energy are more than 1 MeV. In the middle of slab,the energy spectrums get to the max firstly and then drop down with exponential. At the end, the energy spectrums get to the max and then keep a constant. The energy spectrums of multi-scattered-photons are colsed with the rusults from MC code. There is bigger difference between them only in low energy. Conclusion： The comparison results show good similarity between the two methods. The characteristic line method can calculate the flUence distribution of photon efficiently and accurately.