考虑到激光间质热疗时激光从内置在组织中的光纤输出且光束为有限宽光束,建立了内插光纤的双层球体组织模型;基于蒙特卡罗法获得无限窄光束在组织内的吸收值,利用入射光强与格林函数进行卷积计算,获得有限宽光束的光传输方程;以高斯光束和平圆光束为例,分别对比分析了考虑与不考虑内插光纤时组织对有限宽光束吸收情况的变化。结果表明:组织对平圆光束的吸收值较小、对高斯光束的吸收值较大,内插光纤对组织吸收平圆光束的影响较小,而对光子出射中心附近组织吸收高斯光束的影响较大。因此,采用高斯光束进行激光间质热疗时应考虑内插光纤对组织吸收光能的影响。所提模型更加接近激光间质热疗的实际情况,对准确预估激光间质热疗的热毁损范围具有重要的意义。

In this paper, we established a double-layered spherical model of tissues with an inserted optical fiber, considering that a finite-diameter photon beam was emitted from the optical fiber embedded in the tissue during the laser-induced interstitial thermotherapy (LITT). Firstly, based on the Monte Carlo method, we obtained the absorption of an infinitely narrow beam in the tissue. Then, we attained the light transmission equation of a finite-diameter photon beam through the convolution between the incident light intensity and the Green's function. Finally, taking a Gaussian beam and a flat-topped beam as examples, we compared the changes in the absorption of the finite-diameter photon beam with or without considering the inserted optical fiber. The results show that the absorption of tissues for the flat-topped beam is smaller than that for the Gaussian beam. Besides, the influence of the inserted optical fiber on the absorption of the flat-topped beam is smaller, whereas its influence on the absorption of the Gaussian beam near the photon emission center is larger. Therefore, the influence of the inserted optical fiber on the absorption of light energy of tissues should be considered in LITT using the Gaussian beams. The proposed model follows the actual situation of LITT, which is of great significance for accurately predicting the thermal damage range of LITT.