不同治疗床在调强放射治疗和容积旋转放射治疗中的剂量影响分析

Dose attenuation in intensity-modulated radiotherapy and volumetric modulated arc therapy with different treatment couches

目的:分析不同治疗床在调强放射治疗(IMRT)和容积旋转放射治疗(VMAT)中的剂量影响。方法:随机选取6例2015年收治的前列腺癌患者,每例患者分别设计6 MV的5野、7野、9野角度均分的IMRT计划和152o^212o、210o^150o两个弧的VMAT计划。在治疗计划系统(TPS)中分别导入Elekta i BEAM evo Couch、Varian Exact Couch(Standard Couch)、Varian Exact IGRT Couch、Brain LAB imaging Couch的治疗床模型。在Mosaiq网络治疗系统中,统计每例患者每天治疗时的治疗床坐标,分析治疗床的移动范围,在TPS中模拟不同治疗床位置改变情况。分析有、无治疗床及治疗时患者体位与治疗床相对位置变化对患者靶区的剂量学影响。评价参数为计划靶区(PTV)的平均剂量D_(mean)、95%体积照射剂量D_(95)及计划等中心点剂量D_(iso)。结果:治疗床的剂量平均衰减幅度:Elekta i BEAM evo Couch:1.46%~3.13%;Varian Exact Couch(Standard Couch):0.72%~1.63%;Varian Exact IGRT Couch:0.86%~2.02%;Brain LAB imaging Couch:1.51%~3.15%。5F-IMRT:1.24%~3.15%;7F-IMRT:0.73%~1.73%;9F-IMRT:1.39%~2.83%;VMAT:0.72%~1.78%。每天治疗时患者体位与治疗床相对位置变化范围主要为-2^+2 cm。在TPS中模拟治疗床左右各移动2cm,剂量最大变化幅度为0.65%,剂量平均变化幅度均小于0.30%。结论:治疗床在IMRT和VMAT治疗中的剂量衰减明显,必须在TPS中导入相应的治疗床模型,修正治疗床剂量衰减影响。其中,7F-IMRT和VMAT的剂量平均衰减幅度小于5F-IMRT和9F-IMRT。Varian Exact Couch(Standard Couch)、Varian Exact IGRT Couch的剂量平均衰减幅度小于Elekta i BEAM evo Couch和Brain LAB imaging Couch。患者每天治疗时,治疗床与患者体位相对位置易受摆位影响而改变,但其变化对靶区剂量影响相对较小,可以不予考虑。

Objective To analyze the dose attenuation effect of different treatment couches in intensity-modulated radiotherapy （IMRT） and volumetric modulated arc therapy （VMAT）. Methods The 5-field （5F）, 7-field （7F）, 9-field （9F） IMRT plan （6 MV） and 2-arc （152°-212° and 210°- 150°） VMAT plan were designed for 6 patients with prostate cancer randomly selected from patients admitted in 2015. The treatment couch models of Elekta iBEAM evo Couch, Varian Exact Couch （Standard Couch）, Varian Exact IGRT Couch, and BrainLAB imaging Couch were imported into the treatment planning system （TPS）. The coordinates of the treatment couches were recorded for each patient in the Mosaiq system on a daily basis to analyze the shift of the treatment couch and simulate the changes in the position of the treatment couches in the TPS. The dosimetric effect of patient＇s position and the shift of the treatment couch on the target area were analyzed, and the dose distributions with and without treatment couches were calculated. The mean dose, dose covering 95% of the target volume, and the isocenter dose of the planning target volume were evaluated. Results The average dose attenuation was 1.46%-3.13% with Elekta iBEAM evo Couch, 0.72%-1.63% with Varian Exact Couch （Standard Couch）, 0.86%-2.02% with Varian Exact IGRT Couch, and 1.51%- 3.15% with BrainLAB imaging Couch; the attenuation was 1.24%-3.15% in 5F-IMRT, 0.73%-1.73% in 7F-IMRT, 1.39%- 2.83% in 9F-IMRT, and 0.72%-1.78% in VMAT. The shift of the treatment couches relative to the patient＇s position ranged from -2.00 cm to ＋2.00 cm. Simulation of treatment couch shift in the TPS showed that a treatment couch shift by 2 cm to the fight or the left caused a maximum dose change of 0.65% and an average dose change of less than 0.30%. Conclusion The dose attenuation caused by the treatment couch is obvious in IMRT and VMAT, suggesting the necessity of incorporating the accurate couch model in the TPS to minimize dose attenuation. The average dose attenuation is less obvious in 7F-IMRT and VMAT than in 5F-IMRT and 9F-IMRT. Varian Exact Couch （Standard Couch） and Varian Exact IGRT Couch cause less average dose attenuation than Elekta iBEAM evo Couch and BrainLAB imaging Couch. The position of patients relative to the treatment couch, which may vary in daily set-up, does not cause significant impact on the target dose in the treatment.