均整与非均整模式下TrueBeam加速器治疗床对放疗剂量的影响

Effects of carbon fiber couch of TrueBeam accelerator in flattening filter-free and flattening filter modes on radiotherapy dose

目的:探讨均整(FF)与非均整(FFF)模式下瓦里安TrueBeam加速器全碳纤维治疗床对模体中心和表面剂量的影响。方法:将30cm×30cm×20cm的固体水模分别放置于治疗床薄、中、厚段上,模体的中心与加速器等中心重合,德国IBAFC65-G电离室测量等中心的剂量;选取6/10MV光子束FF/FFF模式4档能量,10cm×10cm标准射野,等中心照射,以机架转角0°~80°(间隔10°采样)为参考,计算100°~180°范围与对应角度参考剂量的比值得到对应角度的穿透因子;将EBT3胶片分别置于上述模体表面和底部,对应机架角度为0°和180°,分析相应的百分深度剂量。结果:4档光子束能量下治疗床薄、中、厚段位置穿透因子范围分别为0.9566~1.0000、0.9554~1.0000和0.9548~1.0000,薄中段在6MV-FFF120°时最小,厚段在6MV-FFF130°时最小。与0°照射相比,180°照射6MV-FFF、6MV、10MV-FFF和10MVX射线表面剂量从30.6%、24.1%、18.3%和14.1%分别增加到95.4%、93%、83%和79.6%。结论:治疗床的存在减少肿瘤剂量、增加表面剂量,FFF模式较FF影响更大,在治疗计划系统中加入虚拟床减小了治疗床引起的剂量学影响。

Objective To evaluate the dosimetric effects of carbon fiber couch of Varian TrueBeam accelerator in flattening filterfree(FFF)and flattening filter(FF)modes on the central and surface doses of a phantom.Methods The solid water phantom of 30 cm×30 cm×20 cm was placed on 3 parts(thin,medium and thick)of treatment couch,and the center of the phantom was set at the isocenter of gantry.The isocenter dose was measured with German IBA FC65-G ionization chamber.In this study,6/10 MV photon beam with FF/FFF models,a standard field of 10 cm×10 cm and isocentric irradiation were adopted.Taking the measured dose with beam incidence angle which was changed in every 10°gantry angles between 0°and 80°as reference,the penetration factor was obtained by calculating the ratio between the dose in the range of 100°-180°and the corresponding reference dose.EBT3 Gafchromic film was placed at the surface and bottom of the phantom,with a corresponding gantry angle of 0°and 180°.Finally,the corresponding percent depth doses were analyzed.Results The penetration factor were 0.956 6-1.0000,0.955 4-1.000 0 and 0.954 8-1.000 0 at the thin,medium and thick parts of treatment couch.The penetration factor was minimal at 6 MV-FFF 120°for thin and thick parts of treatment couch,and at 6 MV-FFF 130°for the thick part of treatment couch.Compared with 0°irradiation,180°irradiation with 6 MV-FFF,6 MV,10 MV-FFF and 10 MV X rays increased the surface doses from 30.6%,24.1%,18.3%and 14.1%to 95.4%,93.0%,83.0%and 79.6%,respectively.Conclusion The use of carbon fiber couch not only reduces tumor dose,but also increases surface dose.Moreover,the dosimetric effect of FFF modes is significantly larger than FF mode.The application of virtual carbon fiber couch in treatment planning system can reduce the dosimetric effects caused by treatment couch.