随着适形放疗的普及 ,作为最后一环的出射剂量及其分布的监测和验证的重要性已引起愈来愈多的关注[1— 3] 。由于临床实际操作上的原因 ,在利用证实片或电子射野影像系统作出射剂量分布的测量和照射靶区的验证时 ,一般证实片或影像仪都要离开出射面一段距离。而这时所测得的剂? With the popularity of conformal radiotherapy, the importance of monitoring and validating the exit dose and its distribution as the last ring has drawn increasing attention [1-3]. Due to clinical practical reasons, it is generally confirmed that the slice or imager must be located at a distance from the exit surface during the measurement of the dose distribution and the verification of the irradiation target area by using the verification film or the electron field imaging system. At this time the measured agent?