评价电子射野影像系统验证鼻咽肿瘤放疗摆位误差时对患者剂量的影响

目的：分析利用电子射野影像系统（EPID）纠正鼻咽癌调强放射治疗（IMRT）的摆位误差时，对患者剂量的影响方法：选取IMRT的鼻咽癌患者30例，每拍一次电子射野影像片（EPI），拍摄正位片（机架角为0°）和侧位片（机架角为90°）共计2张。拍摄正、侧位片时射野面积一般取10cm×10cm，机器跳数为3MU-4MU。利用荷兰核通公司的Masterplan三维治疗计划系统（Version 1．5）进行剂量计算统计。结果：每拍一次EPI，机器跳数为3MU，当眼晶体在射野范围之内时，射线是能量为6MV的光子束，左、右眼晶体平均剂量为4．8cGy左右；射线是能量为8MV的光子束，左、右眼晶体平均剂量为4．6cGy左右。脊髓和脑干受照部分的平均剂量为4．7cGy左右。如果拍一次EPI机器跳数为4MU，器官受照部分的平均剂量要相应增加33％。结论：建议在鼻咽癌放疗前，都于EPID下进行实时摆值误差纠正，而在设计鼻咽癌治疗计划时,需将这部分剂量考虑到计划中，以避免剂量误差一更好的方法是在治疗计划中设计拍EPI的射野时，用准直器、MLC保护眼晶体、脊髓和脑干等危及器官。

Evaluate the Dose Affection on Patient in the Process of Validating Set-up Error by Electronic Portal Imaging Device(EPID)for Nasopharyngeal Carcinoma(NPC)

Objective： To analyse the extra dose affection on patient in the process of validating set-up error by electronic portal imaging device （EPID）for nasopharyngeal carcinoma （NPC） intensity modulated radiation therapy （IMRT）. Methods： Thirty patients were enrolled in study.Electronic portal images （EPI） were aquired with field size 10 cm×10 cm, 3 MU-4 MU,the anterior and lateral EPI were included in the radiation therapy course. And then,analyzing the extra dose affection on patient by Oncentra Materplan （Version 1.5SP1,Nuclectron）. Results： When the electronic portal images with 3 MU, 6 MV X-ray were done,the average exposure dose of the left or right lens is about 4.8 cGy. And 8 MV X-ray, about 4.6 cGy on left or right lens, 4.7 cGy on spinal cord and brain stem. If the EPI is done with 4 MU, the average dose of organ at risk（OAR）will increase 33%. Conclusions： So as to validate the set-up error, we suggest that electronic portal image should be required before the NPC radiation treatment. Besides, In designing the treatment plan phase, we should consider the extra dose affection on patient in the EPI process. The best method is that we protect the organ at risk （lens, spinal cord, brain stem, etc.） by jaws or multileaf collimator in the course of EPI.