Objective:To analyze the inter-fractional variations in patient setup during the course of intensity modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC) using kilovoltage cone-beam computed tomography,thus to improve the accuracy of radiotherapy.Methods:64 NPC patients treated with intensity modulated radiotherapy based on nine kV-CBCT scans were included in this study,the position of the patients was studied by kV-CBCT,and the setup errors were recorded and analyzed for guiding the setup in the following treatments.The position data of the 1st to 3rd scans were included into groups A,those of the 4th to 6th scans into group B,and 7th to 9th in group C.The deviations of position and margin of target areas in the three groups were compared statistically.Results:In Group A,the translational errors along the X (right to left),Y (cranial to caudal) and Z (anterior to posterior) axis were (0.58 ± 1.49),(0.82 ± 1.96) and (0.53 ± 1.64) mm respectively,and the corresponding rotation errors were 0.60°± 0.50°,0.82°± 0.58° and 0.62°± 0.53°.In group B,the translational errors along X,Y and Z axis were (0.28±1.08),(0.44±1.35) and (0.11±1.15)mm respectively,and the corresponding rotation errors were 0.68°±0.70°,0.80°±0.52° and 0.63°±0.49°.In group C,the corresponding values were (0.32±1.23),(0.42±1.51) and (0.25t1.24)mm,and 0.61°±0.53°,0.62°±0.53° and 0.60°±0.50°.The margin deviations along X,Y and Z axis were 2.49,3.42 and 2.47mm in group A,1.46,2.05 and 1.08mm in group B,and 1.66,2.11 and 1.49mm in Group C.The translational errors of Group A were higher than those of Group B and group C with statistical difference (P＜0.05),and there were no significant differences of the translational errors between Group B and C (P＞0.05).Conclusion:The records registered by the first three kV-CBCT scans can be used to decrease setup errors in the following intensity modulated radiotherapy for patients with nasopharyngeal carcinoma.%目的:基于千伏级锥形束CT(kV-CBCT)分析鼻咽癌调强放射治疗(IMRT)中分次间摆位误差的变化趋势,提高放疗准确性.方法:64例鼻咽癌患者进行调强治疗,治疗过程中共进行9次kV-CBCT扫描来确定患者的位置,根据前三次摆位误差的规律来指导后续治疗中患者体位的摆放.分别将第1～3、4～6和7～9次kV-CBCT扫描中患者的位置信息设为A、B和C组.比较三组的摆位及靶区边界误差的差异.结果:A组在X(左-右)、Y(头-脚)和Z(前-后)轴方向上的平移和旋转误差分别为(0.58士1.49)、(0.82±1.96)、(0.53±1.64) mm和0.60°±0.50°、0.82°±0.58°、0.62°士0.53°,B组分别为(0.28±1.08)、(0,44±1.35)、(0.11±1.15) mm和0.68°±0.70°、0.80°±0.52°、0.63°士0.49°,C组分别为(0.32±1.23)、(0.42±1.51)、(0.25±1.24) mm和0.61°±0.53°、0.62°±0.53°、0.60°±0.50°.A、B、C三组在X、Y、Z轴上计划靶区的外放边界(MPTV)分别为2.49、3.42、2.47 mm,1.46、2.05、1.08 mm和1.66、2.11、1.49 mm.A组与B、C两组在X、Y、Z轴上的平移误差的差异有统计学意义(P＜0.05),而B、C两组间的误差差异无统计学意义(P＞0.05).结论:在鼻咽癌调强放射治疗中,基于kV-CBCT扫描分析摆位误差的规律,能较好地降低后续治疗中的摆位误差.
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