目的:应用兆伏级锥形束CT分析鼻咽癌调强放射治疗过程中体重变化对摆位精度的影响.方法:15例行逆向调强治疗的鼻咽癌患者,利用三维激光摆位,每周进行体重测量和伏级锥形束CT扫描(采用8MU).采用smoothing H&N滤波核函数进行三维图像重建(矩阵256×256,层厚1mm).将重建的MV-CBCT图像与计划CT进行融合,记录等中心参考点在X、Y、Z坐标轴方向的摆位误差.在每次重建的MV-CBCT图像中都选择颅底和第1、3、5颈椎水平四个层面,测量四个层面的皮肤轮廓在身体左右方向(X轴)、头脚方向(Y轴)、前后方向(Z轴)的距离.以第1次扫描图像作为参考,计算每周皮肤轮廓缩小的距离,分析体重和皮肤轮廓缩小程度与摆位误差的关系.结果:15例患者在放疗过程中体重每周都有下降的趋势,至放疗结束,患者的体重平均下降4.5kg、其中最多消瘦15.2kg.不同层面皮肤轮廓之间的距离逐渐缩小,但第1颈椎和第3颈椎水平皮肤轮廓收缩最明显；颅底和第5颈椎层面的皮肤轮廓距离缩小相对较小.整个治疗过程中,等中心参考点在X、Y、Z坐标轴方向的平均摆位误差分别为0.15mm,0.28mm,0.21mm.随着体重的下降,X、Z轴方向的摆位误差轻微增加(P＞0.05),但摆位误差均≤3mm；而Y轴方向的摆位误差明显增加(P＜0.05),最大误差达到7mm.结论:鼻咽肿瘤患者放疗过程中,体重有逐渐下降的趋势,不同层面皮肤轮廓之间的距离逐渐缩小,其中第1和第3颈椎水平皮肤轮廓收缩最明显；体重下降对X、Z轴方向的摆位误差影响不明显,但对Y轴的影响较大.%Objective:To analyze the influence of weight change to the accuracy of the patient positioning in the intensive modulated radiotherapy (IMRT) of nasopharyngeal carcinoma with mega - volt cone beam CT ( MV -CBCT). Methods: Fifteen patients diagnosed as nasopharyngeal carcinoma were treated with inverse IMRT. With three - dimensional laser to position, patients'weight were recorded and CT scan were made by the MV - CBCT once a week. With the smoothing H&N filter function, the CT image was reconstructed as the matrix of 256 x256 pixels and thick layer of 1 mm. Then the cone beam CT image was fused with the planning CT image and generate the date of the deviation of the isocenter in X, Y, Z direction. In each of the reconstructed MV - CBCT image, we selected the skull base level, the firet, third and fifth cervical vertebra level, and measure the distance within the contour of the body in X, Y, Z axis. Compared with the planning CT images at the same four level, the shrinkage of the body contour could be calculated. Results; In the course of radiotherapy, the weight of 15 patients declined every week, at the end of the therapy, the mean decrease of patients'weight was 4.5kg, the maximum was 15.2kg. As the same, the shrinkage of body contour increased, mostly in the levels of the first and third cervical vertebra, but in the other two levels, the change was relatively small. In the whole treatment course, the mean deviations of the reference isocenter point were 0. 15mm, 0.28mm. 0. 21mm in X, Y, Z axis respectively. With the decline of the body weight, the error of positioning increases slightly, (P > 0.05) which was not more than 3mm averagely, but in Y axis, the error was most obvious, (P <0. 05) the maximum deviation was 7mm. Conclusion; The patients'weight decreases gradually in the course of radiotherapy, and the body contour becomes smaller, the change is most obvious in the levels of first and third cervical vertebras. The positioning of the patient will be influenced by the weight loss, mostly in the Y axis, but to the X and Z axis, the influence is small.
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