鼻咽癌调强放射治疗中心点Y轴精度的临床分析

Y Axis Accuracy of Intensity Modulated Radiotherapy for Nasopharyngeal Carcinoma

目的:探讨鼻咽癌调强放射治疗中心点Y轴精度的情况。方法:2016年1月至5月病理明确诊断鼻咽低分化鳞癌患者50例进入本研究。年龄43~58岁,平均年龄55岁,男女比例为3:1;使用飞利浦CT模拟机进行常规CT螺旋扫描,扫描层厚为5mm;使用Pinnacle放射治疗计划系统制订放射治疗计划,并确保进行研究的50例患者的放射治疗计划所生成的中心是采用原始的中心点即标记点(放疗中心点),重建生成数字重建放射图像(DRR);治疗前使用瓦里安Acuity模拟机(DR)进行验证。模拟机透视记录治疗中心点与CT扫描重建出的DRR图像的中心点确定放射治疗中心点Y轴精度误差发生情况(上移或下移),并分析误差发生的原因。结果:模拟机透视的放疗中心点与DRR图像的中心点相配准后,记录Y轴方向上移或下移的情况。50例患者中,有23例的患者需要上下移动5mm(仅为一个层面),其中12例患者需要上移,11例患者需要下移;27例患者不需移动。模拟机下Y轴中心点精度误差表现为离散均衡,未发现离散至一侧倾向趋势。误差主要发生原因为由于摆位Mark点的几何体积尺寸(1.5mm)和CT扫描层厚(5mm)和CT的扫描方式(螺旋扫描)的原因导致CT扫描中心点(Mark点)出现在临近的上下两层CT扫描层面,使得DRR图像的中心点与模拟机下摆位中心点发生偏差,导致表观上Y轴精度发生误差。结论:CT螺旋扫描会导致Mark点在相邻层面同时出现,导致治疗中心点在Y轴出现误差,治疗前仍需要模拟机验证。

Purpose：To investigate the accuracy of Y-axis based on the isocenter of radiotherapy for nasopharyngeal carcinoma. Methods： From January to May 2016, 50 patients with poorly differentiated nasopharyngeal squamous cell carcinoma were enrolled in this study. Age 43-58 years,mean age 55 years, male to female ratio of 3： 1,The patients were routinely scanned by Philips CT simulator with thickness of 5mm;The radiotherapy plans of 50 patients were generated and Digitally Reconstructed Radiograph （DDR）images were reconstructed in Pinnacle radiotherapy planning system using the original center point as isocenter with the marker （radiotherapy isocenter） the treatment parameters were validated with the Varian Acuity simulator （DR） before treatment. The accuracy of Y-axis （up or down） was determined and analyzed based on the matching error between the isocenter of the DRR images and the isocenter of the simulator. Results： The accuracy in Y-axis direction was determined and documented （moved up or down） based on the matching error between isocenter of the simulator and isocenter of the DRR images. Of the 50 patients, 23 patients needed to move up or down 5mm （only one level）, 12 of whom needed to move up, 11 patients needed to move down,27 patients did not move. The accuracy of the Y-axis with simulator is discrete equilibrium, and tendency to be discretized to one side is found. The main reason for the error in Y-axis was that marker was showed simultaneously on the adjacent CT levels due to the its size （1.5 ram） and the helical scanning thickness （5mm）,which resulted in the matching error between isocenter of DRR images and isocenter of the simulator. Conclusion： Marker was simultaneously on the adjacent CT levels, which was the leading cause of the error in the Y axis. Verification was necessary with simulator before the treatment.