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目的:利用电子射野影像验证系统(electronic portal imaging device,EPID)监测鼻咽癌调强放疗的摆位误差为PTV外扩边界提供依据,使得放射治疗计划的设计更为科学合理。方法:随机选取50例鼻咽癌调强放疗患者,采用“盲拍”模式获取患者治疗前及每周的摆位误差结果,根据公式MPTV=2.5Σ+0.7σ计算CTV到PTV外扩边界。结果:在左右、上下和前后各方向的误差分别为(0.004 6±0.087 5)cm,(0.048 3±0.065 1)cm和(0.027 2±0.137 3)cm;得到左右、上下和前后方向上外扩的边界分别为0.1 cm、0.4 cm、和0.4 cm。结论:通过图像引导方式监测患者的摆位误差情况,可以评估靶区安全的外扩边界。为保证肿瘤区域得到准确的剂量和减少正常组织受量,PTV外扩边界理论上应不小于0.4 cm。
OBJECTIVE: To monitor the setting error of intensity modulated radiotherapy for nasopharyngeal carcinoma (NPC) by using electronic portal imaging device (EPID) to provide the basis for the expansion boundary of PTV, making the design of radiotherapy plan more scientific and reasonable. Methods: Fifty cases of NPC patients undergoing IMRT were randomly selected. The results of the setup error before and after treatment were obtained by using the “Blind” mode. The CTV was estimated to be extrapolated from PTV according to the formula MPTV = 2.5Σ + 0.7σ boundary. Results: The errors in left, right, up and down and front and back directions were (0.0046 ± 0.087 5) cm, (0.048 3 ± 0.065 1) cm and (0.027 2 ± 0.137 3) cm respectively. The expanded boundaries are 0.1 cm, 0.4 cm, and 0.4 cm, respectively. Conclusions: By monitoring the positioning error of the patient through the image guidance method, it is possible to evaluate the safe extended boundary of the target area. In order to ensure accurate tumor area dose and reduce the amount of normal tissue, PTV expansion boundary theory should not be less than 0.4 cm.