目的：应用蒙特卡罗方法建立6711型125I放射性粒子模型，计算其在不同骨介质中的剂量分布，为进一步分析125I放射性粒子治疗头颈部肿瘤的剂量分布提供依据。方法：应用MCNP4C软件（截面库DLC-200）建立6711型125I放射性粒子模型，计算其剂量率常数及在水介质中的径向剂量函数值，并与美国医学物理学家协会（American Association of Physicists in Medicine，AAPM）2004年发布的TG43UI报告推荐值对比验证125I模型。利用该模型，采用ICRU44报告推荐的成人不同骨介质的元素组成及密度编写MCNP材料卡，计算125I模型在不同骨介质中的剂量分布。结果：应用MCNP4C软件建立的125I放射性粒子模型的几何形态及规格符合AAPM的TG43UI要求。模拟得到了125I模型剂量率常数Λ=0.977 78 cGy/(h·U)及在水介质中的径向剂量函数值，并得出125I模型在不同骨介质中的径向剂量分布。在同样的介质深度，125I粒子的剂量沉积能力自骨皮质、骨松质、软骨、水、黄骨髓、红骨髓依次递减。在介质深度分别为0.01、0.1、1 cm处，125I粒子在骨皮质中的剂量沉积分别是在水中的12.90倍、9.72倍、0.30倍。结论：本研究建立的125I放射性粒子模型可靠性好，可用于后续在骨介质中的模拟计算。125I放射性粒子在不同的骨介质中的剂量分布不等同于水，能量主要沉积在骨介质表面，因此在制定靶区毗邻骨组织的125I粒子植入计划时应考虑骨介质的影响。

Objective： To present a theoretical analysis of how the presence of bone in interstitial brachytherapy affects dose rate distributions with MCNP4C Monte Carlo code and to prepare for the next clinical study on the dose distribution of interstitial brachytherapy in head and neck neoplasm. Methods: Type 6711,125I brachytherapy source was simulated with MCNP4C Monte Carlo code whose cross section library was DLC-200. The dose distribution along the transverse axis in water and dose constant were compared with the American Association of Physicists in Medicine (AAPM) TG43UI update dosimetry formalism and current literature. The validated computer code was then applied to simple homogeneous bone tissue model to determine the affected different bone tissue had on dose distribution from 125I interstitial implant. Results: 125I brachytherapy source simulated with MCNP4C Monte Carlo code met the requirements of TG43UI report. Dose rate constant, 0.977 78 cGy/(h·U), was in agreement within 1.32% compared with the recommended value of TG43UI. There was a good agreement between TG43UI about the dosimetric parameters at distances of 1 to 10 cm along the transverse axis of the 125I source established by MCNP4C and current published data. And the dose distribution of 125I photon emitting source in different bone tissue was calculated. Dose-deposition capacity of photons was in decreasing order: cortical bone, spongy bone, cartilage, yellow bone marrow, red bone marrow in the same medium depth. Photons deposited significantly in traversal axis among the phantom material of cortical bone and sponge bone relevant to the dose to water. In the medium depth of 0.01 cm, 0.1 cm, and 1 cm, the dose in the cortical bone was 12.90 times, 9.72 times, and 0.30 times of water respectively. Conclusion: This study build a 125I source model with MCNP4C Monte Carlo code, which is validated, and could be used in subsequent study. Dose distribution of photons in different bone medium is not the same as water, and its main energy deposits in bone medium surface, so we should consider the effect of bone medium when we design the target area adjacent to the bone tissue in 125I sources implantation plan.