鼻咽癌VMAT摆位误差及靶区外扩边界影响因素分析

Analysis of the Influence Factors of Set-up Errors and Planning Target Volume in VMAT for Nasopharyngeal Carcinoma

目的 分析鼻咽癌容积旋转调强放疗(Volumetric Modulated Arc Therapy, VMAT)摆位误差的影响因素及其计划靶区的外扩边界(Margin of Planning Target Volume,MPTV)范围。方法 回顾性分析50例鼻咽癌VMAT患者的临床资料,所有患者采用热塑膜固定,每周行锥形束CT(Cone Beam CT,CBCT)进行图像扫描,并与计划CT图像匹配融合,获得患者左右(x轴)、头脚(y轴)、前后(z轴)3个方向的摆位误差。使用靶区边界外放公式,计算靶区理论外放边界。分别根据患者的体质指数(Body Mass Index,BMI)、年龄进行分组(偏瘦组、正常组和超重组;青年组、中年组和老年组),并分析各组间摆位误差和MPTV的差异。结果 所有患者三维方向(x轴、y轴、z轴)上总体摆位误差分别为(0.198±1.176)、(-0.409±1.607)、(-0.140±1.272)mm,靶区的理论MPTV分别为2.616、3.596、2.615 mm。偏瘦组患者在x轴、z轴方向上摆位误差大于正常组和超重组(P<0.05),其中x轴理论MPTV为3.877 mm。在y轴方向上,超重组患者的摆位误差显著大于偏瘦组和正常组(P<0.05),其理论MPTV为3.988 mm。青年组患者在x、y轴方向上的摆位误差均大于中年组和老年组(P<0.05),其中y轴理论MPTV为3.778 mm。在y轴方向上,年龄与摆位误差呈弱负相关,BMI与摆位误差呈弱正相关(P<0.05),其他指标比较,无明显相关性。结论 鼻咽癌患者接受VMAT治疗期间,虽然年龄、BMI与摆位误差相关性较弱,但各组间的理论MPTV仍存在一定差异,需进一步优化青年、偏瘦和超重患者的体位固定技术以降低摆位误差,还可应用off-line自适应放射治疗技术减少MPTV。

Objective To analyze the influencing factors of set-up errors in volumetric modulated arc therapy(VMAT)for nasopharyngeal carcinoma and the range of margin of planning target volume(MPTV).Methods The clinical data of 50 patients with VMAT nasopharyngeal carcinoma were retrospectively analyzed.All patients were fixed with thermoplastic phantom.Cone beam CT images were scanned weekly,matched and fused with the planned CT images to obtain the set-up errors in the left and right(x-axis),head and foot(y-axis),and front and back(z-axis)directions.The theoretical external boundary of the target area was calculated by using the external boundary formula of the target area.Patients were divided into groups according to body mass index(BMI)and age(lean group,normal group and overweight group;young group,middle-aged group and elderly group).The differences in set-up errors and MPTV among different groups were analyzed.Results The overall set-up errors of all patients in the three-dimensional direction(x-axis,y-axis,z-axis)were(0.198±1.176),(-0.409±1.607)and(-0.140±1.272)mm,respectively,and the theoretical MPTV of the target area were 2.616,3.596 and 2.615 mm,respectively.The set-up errors in the x-axis and z-axis of the lean group were greater than those of the normal group and overweight group(P<0.05),and the theoretical MPTV of x-axis was 3.877 mm.The set-up errors in the y-axis of the overweight group were greater than those of the lean group and normal group(P<0.05),and the theoretical MPTV was 3.988 mm.The set-up errors in the x-axis and y-axis of the young group were greater than those of the middleaged group and the elderly group(P<0.05),and the theoretical MPTV of y-axis was 3.778 mm.In the y-axis direction,age was weakly negatively correlated with set-up error,while BMI was weakly positively correlated with set-up error(P<0.05).There was no significant correlation between other indicators.Conclusion During VMAT treatment,although the correlation between age,BMI and set-up errors is weak,there are still some differences in theoretical MPTV among different groups.It is necessary to optimize the postural fixation techniques for young,lean and overweight patients to reduce set-up errors.Off line ART technology can be used to reduce the tumor target area-MPTV.