Objective We have monitored the set-up errors of radiotherapy for th oraci c cancer using ontime imaging system. The discrepancy of couch-top between tr eatment machine (net-bracket) and simulator (plate) was observed. Set-up error s were analyzed quantitively according to variable distribution, ratio and extent of displacement. Methods Three hundred eighty-nine orthogonal portal images of 30 patients were compared to the simulation films. Set-up deviation in lateral , longitudinal and vertical direction were measured respectively. The systematic and random errors were evaluated. Results The distribution of set-up errors in e ach direction approximated a normal distribution. The systematic error in latera l and longitudinal was lower with a mean of 1～2 mm, and however, it was -6.8 m m in vertical direction without immobilization device as compared to -1.5 mm w ith im mobilization device. The random error was larger and ratio of vertical displacem ent was higher without immobilization. Conclusions The decline of treatment couc h-top (with net-bracket) could result in a larger systematic error with a mea n o f 6.8 mm. We must pay attention to the coordination of couch-top between treatm e nt machine and simulator when radiation field is set. The use of immobili zation device during thoracic radiotherapy is able to improve the geometrical qu ality.%目的 使用实时照射野成像系统监测胸部放射治疗时的摆位误差,定量分析治疗机床面(网状支 架)和模拟机床面(平板)不一致时误差的分布情况、移位的几率和程度,并评价 固定器的价值。 方法 用正交摄影法随机采集30例胸部肿瘤放射治疗时的正侧位照射野 图像共计389幅,并与同期相应的模拟定位片相比较,分别测量解剖参考点的纵 向、横向和垂直方向的移动度。 结果 各方向摆位误差的分布均近似正态分布,横向和纵 向的系统误差值较小(大约为1～2?mm),随机误差相似。垂直方向无固定组的系统 误差值平均为－6.8?mm,固定组为－1.5?mm,两组间差异有显著性意义( u =6 .07,P=0.000)。无固定组的随机误差较大,垂直下陷的几率显著高于 固定组 (χ2=12.36,P=0.001)。 结论 治疗床面的下垂是造成系统 摆位误差的 一个重要原因,必须及时加以纠正,使用适当的固定器有助于提高摆位精度。
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