Objective To study the dosimetric changes in the organs-at-risk (OAR) caused by setup errors and anatomical changes in patients receiving radiotherapy for nasopharyngeal carcinoma (NPC) and provide evidence for further adaptive radiotherapy.Methods Ten NPC patients undergoing intensity-modulated radiotherapy (IMRT) were selected randomly for analysis.In 6 sessions of IMRT,kV-cone beam CT (CBCT)-guided position correction was performed and the set-up errors of the entire treatment course of treatment were recorded.After registration between the original CT and CBCT images by incorporating the shift data,the actual position of the patient was obtained during the therapy.Using the parameters identical to those in the original IMRT,simulated delineation of the target areas and OAR was performed on the CBCT images,and the deformable registration was used to calculate the accumulative doses to compare the dosimetric variations in the OAR including the parotid glands,spinal cord,and brainstem.Results A total of 60 CBCT scans were performed in the 10 NPC patients during the course of radiotherapy,and the mean setup errors in the X,Y,and Z axis were (-0.883±2.084),(0.567±1.320),and (-0.233±1.661) mm,respectively;the setup errors in the X,Y,and Z axis were no greater than 3 mm in 90.0％,98.3％,and 98.3％ of the scans,respectively.The mean deviation of the maximum dose (Dmax) was 1.320％ (-11.290％-9.620％) in the brainstem and 3.353％ (-4.250％-13.880％) in the spinal eord-Dmax;the volume covered by 3 000 cGy (V30) was 6.777％ (-57.950％-112.330％) in the left and 19.937％ (-72.560％-177.980％) in the right parotid gland;the mean dose (Dmean) was 4.022％(-17.730％-42.000％) in the left and 7.634％ (-19.860％-39.950％) in the right parotid gland.Conclusion The set-up errors and anatomical changes of the NPC patients affect the dose in the OAR during IMRT.A dose deviation from the planned dose in the OAR (especially the parotid glands) occurs during the latter half of IMRT sessions,and dose tracking during IMRT provides evidence for determining the time points for individualized adjustment of the treatment plan.%目的:研究鼻咽癌患者放疗过程中,摆位误差和解剖结构的变化所引起的危及器官的剂量学变化,从而为下一步的自适应放疗提供一定依据.方法:随机选取10例进行调强放射治疗的鼻咽癌患者,在放射治疗过程中选取6次进行千伏级锥形束CT(kV-CBCT)在线引导体位校正,统计整个疗程的摆位误差数据.将每次获得的CBCT图像与CT图像进行中心点的图像配准,并将移位数据带入其中,得到患者治疗时的真实位置,通过模拟计算在保持原始照射条件不变的情况下,在CBCT图像上重新计算,重新勾画靶区及危及器官,然后通过形变配准进行剂量的叠加,比较腮腺、脊髓、脑干等危及器官的剂量学变化.结果:(1)10例鼻咽癌患者在放射治疗过程中一共进行了60次的CBCT扫描,其摆位误差数据在X,Y,Z轴分别为:(-0.883±2.084),(0.567±1.320),(-0.233±1.661)mm,其中X轴方向90％≤3 mm,Y轴方向98.3％≤3 mm,Z轴方向98.3％≤3 mm.(2)正常组织的平均剂量偏差:脑干Dmax(最大剂量)为1.32％(-11.29％～9.62％),脊髓Dmax为3.353％(-4.25％～13.88％),左腮腺V30(3 000 cGy的剂量所占的体积)为6.777％(-57.95％～112.33％),右腮腺V30为19.937％(-72.56％～177.98％),左腮腺Dmmean(平均剂量)为4.022％(-17.73％～42％),右腮腺Dmean为7.634％(-19.86％～39.95％).结论:在患者治疗过程中,摆位误差和解剖结构的变化影响危及器官的受量.在适当进行CBCT扫描和在线摆位校正后,通过形变配准和剂量叠加对患者实际受照剂量进行模拟.经研究发现在疗程后半段,患者危及器官尤其是腮腺的剂量与原治疗计划有一定偏差.对患者的剂量追踪可提供患者个性化适合计划调整的时间节点,具有重要的临床参考价值.
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