鼻咽癌IMRT中实际机架角度对计划执行准确度影响

Impact of actual gantry angle on accuracy of intensity-modulated radiotherapy for nasopharyngeal carcinoma

目的 探讨鼻咽癌IMRT中实际机架角度对治疗计划执行准确度的影响,以寻求减少这种影响的方法。方法 应用Pinnacle计划系统对27例鼻咽癌患者设计IMRT计划,每个计划在0-360°范围以30°为间隔选取8个射野。按照射野参数分为A、B两组计划,A组设置最小子野面积5 cm^2和最小跳数5 MU,最大子野数80;B组设置最小子野面积8 cm^2和最小跳数8 MU,最大子野数60。用Mapcheck2分别在归零和实际角度验证射野的γ通过率（3%/3 mm、2%/2 mm）,比较两种方式验证结果差别（Δ值）,分析Δ值与射野角度及计划设计射野参数之间关系。配对样本t检验组间差异。结果 A组计划采用3%/3 mm标准时Δ平均值为0.90%,除270°、300°、330°外其他角度两种验证方式结果比较均不同（P=0.000-0.007）;采用2%/2 mm标准时Δ平均值为2.72%,除300°外均不同（P=0.000-0.019）。B组计划采用3%/3 mm标准时Δ平均值为0.40%,除300°外其他角度两者比较均相近（P=0.052-0.639）;采用2%/2 mm标准时Δ平均值为1.52%,除210°、240°、270°、300°外均相近（P〉0.05）。采用B组计划参数设计时Δ明显减少,两种方式验证结果更加吻合,治疗计划的执行准确度提高。结论 与归零角度验证方式相比,重力等因素会导致实际角度验证的γ通过率降低,这种影响的大小与射野角度无明显的相关性但与计划设计时射野的设置参数有关。在设计鼻咽癌IMRT计划时应尽可能减少总子野数目、增大最小子野面积及最小跳数,提高治疗计划执行准确度。

Objective To investigate the impact of actual gantry angle on the accuracy of intensity-modulated radiotherapy （IMRT） for nasopharyngeal carcinoma （NPC）. Methods A total of 27 patients with NPC were enrolled in this study. IMRT plans were designed with Pinnacle treatment planning system （TPS）,and 8 beams with an interval of 30°（within 0°-360°） were selected for each plan. These plans were divided into plan A and plan B according to the beam parameters. In plan A,the minimum sub-field area was 5 cm^2,the minimum number of sub-field monitor unit （MU） was 5,and the maximum sub-field number was 80;in plan B,the minimum sub-field area was 8 cm^2,the minimum number of sub-field MUs was 8,and the maximum sub-field number was 60.The gamma passing rate using the criteria of 3%/3 mm and 2%/2 mm at actual and zero degree gantry angles were calculated using Mapcheck 2 device for dose verification,and were compared with the paired t-test. The relationship between the above differences （Δ value） and the beam angle or the beam parameters was also analyzed. Results In plan A with the criteria of 3%/3 mm,the beams were significantly different （P=0.000-0.007） except for at angles of 270°,300°,and 300°,and the mean Δ value was 0.90%;under the criteria of 2%/2 mm,all beams were significantly different （P=0.000-0.019） except for at an angle of 300°,and the mean Δ value was 2.72%.In plan B with the criteria of 3%/3 mm,the beams showed no significant difference （P=0.052-0.639） except for at an angle of 300 °,and the mean Δ value was 0.40%;under the criteria of 2%/2 mm,all beams showed no significant difference （P〉0.05） except for at angles of 210°,240°,270°,and 300°,and the mean Δ value was 1.52%.When the plan B parameters were used,the Δ value was reduced;the results of two verification methods were more consistent,so the accuracy was also improved. Conclusions Compared with the validation method at zero degree gantry angle,the validation method at an actual gantry angle is associated with reduced gamma passing rate because of some factors such as gravity,which is not significantly related to the beam angle,but the beam parameters. In the design of IMRT plans for NPC,the total number of sub-fields should be decreased as far as possible,and the minimum sub-field area and the minimum number of MU should be increased,so as to improve the accuracy of treatment plans.