Study of detector measuring distal shape and position using range scanner in proton therapy

利用塑料闪烁板、CCD相机和楔形距离移位器（WRS），制作了一种能够测量距离扫描技术中深度剂量分布的远端形状和远端位置探测器。在筑波大学质子医学研究中心（PMRC）的2^<和>照射室，我们采集了有关质子束剂量谱特征的基本数据。测量质子束流线上剂量监测器（M1和M2）记录的电荷，等中心上参考剂量计（M3）设置的电荷，以及M3后面闪烁板（SciCCD） CCD元件上的电荷，我们比较了SciCCD和参考M3得到的深度剂量分布。它们揭示了一种淬灭效应；在布拉格峰和从远端边缘测量的20毫米高LET区域，光产率最大降低15%。这种效应在50mm宽的SOBP区域和由丸和准直器产生的深度剂量分布区域也得到了证实。通过比较SciCCD和M3探测器获得的深度剂量分布，以及在治疗计划程序中使用铅笔束算法（PBA）计算的深度剂量分布，我们揭示了分布的详细形状的差异，以及更精确的定位方法和淬火效应校正的重要性。在WRS系统上安装自动控制系统有望提高数据采集速度，并提供各种剂量分配。将闪烁板和CCD相机应用于PMRC的等焦验证系统。该系统将测量时间提高到30分钟，是传统方法的四分之一，精度不降低±0.1 mm。

Using a plastic scintillating plate, a CCD camera, and a wedge-type range shifter(WRS), we produced a distal-shaped and distal-position detector capable of measuring depth dose distribution in range-scanning technique. At the 2^<nd> exposure room in the proton medical research center (PMRC), University of Tsukuba, we took basic data concerned with characteristics of dose profiles in the proton beam. Measuring charges recorded on the dose monitors (M1 and M2) in the proton beam line, charges on the reference dose meter (M3) setup on the isocenter, and charges on the CCD elements viewing the scintillating plate (SciCCD) just behind M3, we compared the depth dose distributions obtained by SciCCD to those done by the reference M3. They revealed a quenching effect; 15% light-yield reduction at maximum at the Bragg peak and in the high LET regions of 20-mm measured from the distal edge. This effect was confirmed as well as in the regions of the 50-mm width SOBP and the depth dose distribution produced with the bolus and collimator. In the comparison among the depth dose distributions obtained using SciCCD and M3 detectors, and calculated with the pencil beam algorithm (PBA) in the treatment planning program, we revealed difference in detailed shapes of the distribution and importance of more accurate positioning methods as well as correction of quenching effects. Installing an automatic control system to the WRS system is expected to improve data taking speeds and to provide various kinds of dose distribution. The scintillating plate and the CCD camera were applied to the isocenter verification system at PMRC. This system improved measurement time to 30-min, the one-forth of the conventional method, without degradation in accuracy of ±0.1 mm.