低能X射线术中放疗系统剂量学特点和潜在临床应用局限性

Intraoperative radiotherapy using low-energy X rays：dosimetric characteristics and potential limitations in clinical application

目的：测试移动式光子线术中放疗德国INTRABEAM系统，分析X射线源及球形施源器下低能光子线剂量学特点和潜在临床应用局限性。方法采用专用水箱、平行板电离室和静电计等测量仪器，对INTRABEAM系统X射线源和不同球形施源器深度剂量率，x、y平面剂量分布各向同性等进行测量，并与系统数据进行比对。结果 X射线源深度剂量率测量值与系统数据偏差为（－2．16±1.36）％，x、y平面各向同性的相对偏差为－1．9％～2.1％。不同直径施源器的深度剂量率测量值、转换系数和 x、y平面各向同性与系统数据偏差范围分别为－10．0％～2.3％、－8．9％～4.2％和－1．6％～2.6％。球形施源器直径越小表面剂量率越高，剂量梯度越大。 X射线源、球形施源器的深度剂量率、各向同性的测量重复性较好。影响测量准确度的因素包括电离室的位置偏差、能量响应、噪声电流和修正因子f′（ R）等。结论通过测试可充分了解INTRABEAM剂量特性、验证系统数据准确性，获得临床应用和日常质量保证所需数据，但剂量梯度大、治疗范围局限等限制其临床广泛应用。

Objective To test intraoperative radiotherapy with mobile photon beam using the INTRABEAM system （ Germany） , and to analyze the dosimetric characteristics of low-energy photon beam using X-ray source and spherical applicators and explore its potential limitations in clinical application. Methods A special water phantom, a parallel-plate ionization chamber, and an electrometer were used to measure the depth dose rates and isotropy of dose distribution in x/y plane of X-ray source and different spherical applicators in the INTRABEAM system. Those data were then compared with the system data. Results For the X-ray source, the deviation of observed depth dose rate and isotropy in the x/y plane from the system data were-2.16%± 1. 36% and-1.9%~ 2. 1%, respectively. For applicators with different diameters, the deviation of observed depth dose rate, transfer coefficient, and isotropy in x/y plane from the system data were-10.0%~2. 3%,-8.9%~4. 2%, and-1.6%~2. 6%, respectively. Surface dose rate and dose gradient became larger with the decrease in the diameter of the spherical applicator. The measurement of depth dose rate and isotropy of X-ray source and spherical applicators showed good repeatability. The influencing factors for measurement accuracy included the positioning error of ionization chamber, energy response, noise current, and correction factor f ’ （ R ） . Conclusions This study reveals the dosimetric characteristics of the INTRABEAM system, verifies the accuracy of the system data, and obtains the data for clinical application and routine quality assurance. However, large dose gradient and small therapeutic range may limit its wide clinical application.