Fielding Andrew L, Evans Philip M, Clark Catharine H
Joint Department of Physics, Institute of Cancer Research and Royal Marsden Hospital NHS Trust, Downs Road, Sutton, Surrey SM2 5PT, UK.
Int J Radiat Oncol Biol Phys. 2002 Nov 15;54(4):1225-34. doi: 10.1016/s0360-3016(02)03749-5.
The precise shape of the three-dimensional dose distributions created by intensity-modulated radiotherapy means that the verification of patient position and setup is crucial to the outcome of the treatment. In this paper, we investigate and compare the use of two different image calibration procedures that allow extraction of patient anatomy from measured electronic portal images of intensity-modulated treatment beams.
Electronic portal images of the intensity-modulated treatment beam delivered using the dynamic multileaf collimator technique were acquired. The images were formed by measuring a series of frames or segments throughout the delivery of the beams. The frames were then summed to produce an integrated portal image of the delivered beam. Two different methods for calibrating the integrated image were investigated with the aim of removing the intensity modulations of the beam. The first involved a simple point-by-point division of the integrated image by a single calibration image of the intensity-modulated beam delivered to a homogeneous polymethyl methacrylate (PMMA) phantom. The second calibration method is known as the quadratic calibration method and required a series of calibration images of the intensity-modulated beam delivered to different thicknesses of homogeneous PMMA blocks. Measurements were made using two different detector systems: a Varian amorphous silicon flat-panel imager and a Theraview camera-based system. The methods were tested first using a contrast phantom before images were acquired of intensity-modulated radiotherapy treatment delivered to the prostate and pelvic nodes of cancer patients at the Royal Marsden Hospital.
The results indicate that the calibration methods can be used to remove the intensity modulations of the beam, making it possible to see the outlines of bony anatomy that could be used for patient position verification. This was shown for both posterior and lateral delivered fields.
Very little difference between the two calibration methods was observed, so the simpler division method, requiring only the single extra calibration measurement and much simpler computation, was the favored method. This new method could provide a complementary tool to existing position verification methods, and it has the advantage that it is completely passive, requiring no further dose to the patient and using only the treatment fields.
调强放射治疗所产生的三维剂量分布具有精确的形状，这意味着患者体位和摆位的验证对治疗结果至关重要。在本文中，我们研究并比较了两种不同的图像校准程序的使用情况，这两种程序可从调强治疗射束的测量电子射野图像中提取患者的解剖结构。
获取使用动态多叶准直器技术输送的调强治疗射束的电子射野图像。这些图像是通过在射束输送过程中测量一系列帧或段而形成的。然后将这些帧相加，以生成输送射束的积分射野图像。为了去除射束的强度调制，研究了两种不同的积分图像校准方法。第一种方法是将积分图像逐点简单地除以输送到均匀聚甲基丙烯酸甲酯（PMMA）体模的调强射束的单个校准图像。第二种校准方法称为二次校准法，需要一系列输送到不同厚度均匀PMMA块的调强射束的校准图像。使用两种不同的探测器系统进行测量：瓦里安非晶硅平板成像器和基于Theraview相机的系统。在获取皇家马斯登医院癌症患者前列腺和盆腔淋巴结的调强放射治疗图像之前，首先使用对比体模对这些方法进行测试。
结果表明，校准方法可用于去除射束的强度调制，从而能够看到可用于患者体位验证的骨骼解剖轮廓。这在后方和侧方输送野中均得到了证明。
观察到两种校准方法之间差异很小，因此更简单的除法方法（仅需要单次额外的校准测量且计算简单得多）是首选方法。这种新方法可以为现有的体位验证方法提供一种补充工具，其优点是完全被动，无需对患者进一步施加剂量，仅使用治疗野。
