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背景与目的目前适形和调强放射治疗已逐渐用于鼻咽癌,与传统放疗相比,适形及调强放疗对摆位精确度要求更高。随着放射治疗向高精度方向发展,研究放射治疗中的摆位误差具有重要意义,由于采用的设备及技术员的素质不同,各个治疗中心需要有自己的摆位误差数据。为此,本研究采用射野验证片测量本院鼻咽癌适行放射治疗中的摆位误差,并计算出PTV的外扩范围。方法20例行三维适行放射治疗的鼻咽癌患者,所有患者都用面罩固定,放射治疗过程中每周拍射野验证片,通过比较DRR和验证片上骨性标志与射野的相对位置来计算摆位误差。根据所测得的摆位误差数据计算CTV到PTV应该预留边界的大小。结果20例患者,共摄取验证片240张射野片,有22张射野片因为无法辨认出两个或两个以上解剖结构而被剔除。将各方向的随机和系统误差分开研究。左右,头脚,前后各向总误差为2.8、2.7、2.8mm,系统误差分别为2.4、2.3、2.4mm,随机误差分别为1.4、1.5、1.5mm,移动均值分别为-1.1,-0.1,-0.25mm。缩野前后左右方向上的系统误差分别为2.3、2.5mm,随机误差分别为1.2、1.5mm;头脚方向的系统误差分别为2.2、2.4mm,随机误差分别为1.4、1.6mm;前后方向上的系统误差分别为2.4、2.5mm,随机误差分别为1.3、1.4mm。三维方向上摆位偏移大于3mm的百分数26.3%,大于4mm的百分数15.1%,大于5mm的百分数6.5%。单一方向上平均摆位偏移大于3mm的百分数,左右17.5%,头脚20%,前后22.5%。比较每位患者缩野前后各个方向上系统误差和随机误差,仅左右方向上缩野后随机误差有增大,其他各方向上缩野前后系统和随机误差均无显著性差异。据Stroom等推荐的公式CTV到PTV边界至少应为2Σ+0.7σ来计算各个方向上CTV到PTV应该预留的边界大小,3个方向上CTV与PTV间预留间隙6mm是可行的。结论在鼻咽癌的三维放射治疗中,用我科的固定及摆位技术,CTV与PTV间预留间隙6mm是可行的。缩野前后除左右方向上随机误差有显著差异外,其他各方向系统误差和随机误差没有显著差别。缩野时重新制作面罩,特别是体重下降比较明显的患者是否能降低放射治疗中的摆位误差值得近一步研究。
BACKGROUND & OBJECTIVE: Currently, conformal and intensity modulated radiation therapy has been gradually applied to nasopharyngeal carcinoma. Compared with traditional radiotherapy, conformal and intensity modulated radiotherapy require more precision in positioning. With the development of radiotherapy toward high precision, it is of great significance to study the setup error in radiotherapy. Due to the different qualities of equipments and technicians used, each treatment center needs to have its own setup error data. To this end, this study uses field verification tablets measurement of nasopharyngeal carcinoma in our hospital suitable positioning error in radiotherapy, and calculate the expansion of PTV range. Methods Twenty patients with nasopharyngeal carcinoma undergoing three-dimensional radiotherapy were enrolled. All the patients were fixed with masks. Radiological verification was made every week during radiotherapy. By comparing the DRR and verifying the relative position of the on-chip bony signs and the field Calculate the setup error. Calculate the size of the border between CTV and PTV based on the measured setup error data. Results A total of 240 shots were taken from 20 patients and 22 shots were rejected because two or more anatomical structures were not identified. Separate the random and systematic errors in all directions. Left and right, head and foot, before and after the total error of 2.8,2.7,2.8 mm, the system errors were 2.4,2.3,2.4 mm, respectively, the random errors were 1.4,1.5,1.5 mm, the moving average were -1.1, -0.1, -0.25mm. The system errors in the left, right, left and right direction of shrinkage were 2.3 and 2.5 mm respectively, and the random errors were 1.2 and 1.5 mm respectively. The systematic errors in the head and foot directions were 2.2 and 2.4 mm respectively, and the random errors were 1.4 and 1.6 mm respectively. The systematic errors are 2.4 and 2.5 mm respectively, and the random errors are 1.3 and 1.4 mm respectively. The percentage deviation of more than 3mm in three-dimensional direction was 26.3%, the percentage of more than 4mm was 15.1% and the percentage of more than 5mm was 6.5%. The average deviation in a single direction is greater than the percentage of 3mm, about 17.5%, 20% head and foot, before and after 22.5%. The systematic errors and random errors in each direction before and after the zoom-out of each patient were compared. Only the random errors after shrinking in the left-right direction increased, and there was no significant difference in the other systems before and after the shrinking. According to the formula recommended by Stroom et al., The boundary between CTV and PTV should be at least 2Σ + 0.7σ to calculate the boundary between CTV and PTV in all directions. It is feasible to reserve a gap of 6mm between CTV and PTV in all three directions. Conclusion In the three-dimensional radiotherapy of nasopharyngeal carcinoma, with our fixed and positioning technique, a gap of 6mm between CTV and PTV is feasible. There was no significant difference between the systematic errors and the random errors in other directions except for the significant differences in random errors in the left and right directions before and after the reduction. It is worth further studying whether the resurfacing mask should be made during the shrinking of the filed, especially if the patients with obvious weight loss can reduce the setting error in radiotherapy.