基于光学表面监测系统对头部肿瘤放疗中摆位误差的研究

Study on setup errors in radiotherapy for head tumors based on an optical surface monitoring system

摘要:

目的比较分析头部肿瘤两种固定技术的摆位误差，评估以光学表面监测系统（optical surface monitoring system，OSMS）为辅助手段引导摆位并进行监测的可行性。方法头部肿瘤患者A组26例采用发泡胶+头部热塑闭合面罩固定，B组20例采用发泡胶+头部热塑开口面罩固定+光学表面监测系统。A组摆位后进行锥形束CT( conebeam computed tomography, CBCT )扫描共178次，B组摆位经过OSMO调整后进行治疗前CBCT扫描162次，将获得的CBCT图像和计划CT图像匹配，得到X、Y、Z轴方向上的平移及旋转误差，并对摆位误差进行比较分析。通过OSMS调整摆位误差的标准为三维方向上平移误差≤ 1mm、旋转误差≤1°，调整后OSMS监测的摆位误差与治疗前CBCT 扫描配准摆位误差的差异定义为OSMS的精度。运用OSMS跟踪监测整个治疗过程中患者的实时摆位误差，并记录分次内的最大三维矢量误差、旋转误差及治疗时间。结果B组与A组分次间的三维矢量误差（1.73±0.90）mm/（2.24±1.25）mm，X、Z轴上的平移误差（0.04±1.00）mm/（-0.34±1.48）mm、（0.06±0.86）mm/（0.29±1.14）mm，X轴旋转角度误差（0.01±0.09）°/（0.18±0.95）°和平均摆位时间2.5 min/2.0 min,经比较差异具有统计学意义（P＜0.05)。B组OSMS的精度在三维矢量误差为（1.28±0.74）mm。OSMS跟踪监测患者分次内的最大三维矢量误差（0.71±0.47）mm,旋转角度误差为（0.46±0.40）°。结论与常规的采用闭合面罩固定相比，采用开口面罩固定并应用OSMS辅助摆位，不仅可便捷有效地减小头部肿瘤患者的分次间摆位误差，而且能实时跟踪并监测分次内的摆位误差。

关键词:头部肿瘤|光学表面监测系统|开口面罩|锥形束CT|摆位误差

Abstract:

ObjectiveTo evaluate the feasibility of using an optical surface monitoring system (OSMS) as an auxiliary means to guide placement and monitoring in head tumor radiotherapy by comparative analysis of setup errors with two fixation techniques.Methods26 patients with head tumors in group A were immobilized using polyurethane foam sealing agent pillow with closed head thermoplastic masks and 20 in group B using polyurethane foam sealing agent pillow with open head thermoplastic masks plus an OSMS. Before treatment, a total of 178 cone beam computedtomography（CBCT）scans were performed after setup in group A, and 162 scans after OSMO adjustment in group B . The obtained CBCT images were matched with the planned CT images, and the translation and rotation setup errors along the X, Y and Z axes were obtained. The standards of adjusting setup errors by OSMS were translation errors in threedimensional direction (≤1 mm) and rotation errors(≤1 °). The differences between setup errors by OSMS monitoring after adjustment and those of CBCT scanning registration were defined as the accuracy of OSMS. OSMS was used to track and monitor the realtime setup errors of patients in the whole course of treatment, and the maximal threedimensional vector errors, rotation errors and setup time were recorded during intrafractional treatment.ResultsThe threedimensional vector errors of group B and group A were (1.73±0.90) mm and (2.24±1.25) mm, the translation errors on X and Z axes were (0.04 ±1.00) mm/(-0.34 ±1.48) mm and (0.06 ±0.86) mm/(0.29 ±1.14) mm, the rotation angle errors on X axis were（0.01°±0.09°） and （0.18°±0.95°), with the average setup time of 2.5 minutes and 2.0 minutes. The differences were all statistically significant (all P<0.05). The accuracy of OSMS in group B was (1.28±0.74) mm in three dimensional vector error. The maximal threedimensional vector error was (0.71±0.47) mm and the rotation angle error was (0.46°±0.40°) during intrafractional treatment.ConclusionCompared with the conventional closed mask fixation, the open mask fixation and OSMSassisted positioning can not only reduce the interfractional setup errors of head tumor patients conveniently and effectively, but also track and monitor the intrafractional setup errors in real time.

Key words:Head tumor|Optical surface monitoring system|Open head thermoplastic mask|Cone beam computed tomography|Setup error