比较两种CT模拟定位方法在调强放疗中的误差影响

目的 比较调强放疗中CT模拟定位最终等中心标记法和参考点标记法的误差。方法2009-2012年间327例头颈部肿瘤调强放疗患者使用Philips BrillianceCT Big Bore扫描机及Philips Tumor LOC模拟定位工作站进行定位，其中最终等中心标记法、参考点标记法定位患者分别为208、119例。在瓦里安Eclipse治疗计划系统进行勾画靶区和设计治疗计划。治疗前使用瓦里安EX 直线加速器的机载影像系统千伏级锥形束CT扫描和配准，得出左右、上下、前后方向误差后进行两组比较并成组t检验。最后对5例鼻咽癌患者通过治疗计划系统测量等中心在3个方向移动后对危及器官剂量的影响。结果 最终等中心标记法组3个方向平均误差均小于参考点标记法组(P=0.02、0.01、0.03)。等中心在3个方向上移动后的剂量比较结果显示，靶区受量降低的同时靶区周围正常组织受量明显提高，前后方向误差对脊髓和脑干影响最大。结论 CT模拟定位最终等中心标记法的误差优于参考点标记法，等中心移动的较小误差可引起危及器官较大的受量变化。

Errors of two CT simulation positioning methods in intensity-modulated radiotherapy: a comparative study

Objective To compare the errors of final isocenter marking method and reference pointmarking method for CT simulation positioning in intensity-modulated radiotherapy (IMRT). MethodsFrom 2009 to 2012, 327 patients with head and neck cancer for IMRT underwent CT simulation positioning using the Philips Brilliance CT Big Bore scanner and Philips Tumor LOC workstation and were divided into final isocenter marking group (n=208) and reference point marking group (n=119) according to positioning methods. Target volume delineation and treatment plan design were performed on the Varian Eclipse treatment planning system (TPS). Before treatment, kilovoltage cone-beam CT scans and registration were performed with the Varian EX on-board imager system to obtain beam position errors in the right-left (RL), superior-inferior (SI), and anterior-posterior (AP) directions, and then comparisons of errors between the two groups were made by independent-samples t test. Finally, the TPS was used to measure the changes in the doses to the organs at risk after moving isocenters in the RL, SI, and AP directions among 5 patients with nasopharyngeal carcinoma. Results The mean beam position errors in the three directions were less in the final isocenter marking group than in the reference point marking group (P=0.02,0.01,0.03). After moving isocenters in the three directions, the target dose was reduced and the dose to the normal tissue around the target tumor was increased significantly. The error in the AP direction had the maximum influence on the spinal cord and brainstem. Conclusions Final isocenter marking method leads to less beam position error than reference point marking method in CT simulation positioning. Small isocenter motion can cause large changes in the doses to the organs at risk.