Application of double scattering method using a dual-ring to formation of irradiation field for proton cancer therapy

阐明了一种将双环第二散射体的高效双散射方法应用于质子癌症治疗的方法。由于插入诸如距离移位器和距离调制器(条脊滤光器)的束流修正装置来改变横向剂量分布，因此不可能通过单个散射体组合来覆盖用于癌症治疗的整个深度范围。结果表明，将辐射覆盖深度划分为8个射程，并对每个射程采用适当的第一和第二散射体组合，可以得到整个深度区均匀的横向剂量分布，阐明了将该方法应用于发射度较大的束流的方法。通过改变三个自由参数来近似优化描述横向剂量分布的六个参数，使这六个参数接近于零发射度情况的最优解。结果还表明，在发射度相同的情况下，剂量分布对束流相空间参数的选择非常敏感。聚焦、发散或会聚的光束会产生不同的剂量分布，由于该方法对光束不对准很敏感，因此有必要对分布的平坦度进行监测。为此，我们制作了一个电离室，在一个平面的四个象限上放置了四个相同的收集电极，每个电极都与其他电极电隔离。我们对脉冲质子束(5秒半高宽)的监测室进行了测试，发现它可以用来监测小于1%的不均匀性。通过研究，我们阐明了这种方法可以有效地用于质子癌症的治疗。

A method of applying the efficient double scattering method using a dual-ring 2nd scatterer to proton cancer therapy has been clarified. Since insertion of beam modifying devices such as a range shifter and a range modulator (a bar ridge filter) modifies the lateral dose distribution, it is impossible to cover the whole depth range used for cancer treatment by a single scatterer combination. It is shown that we can obtain the uniform lateral dose distribution for the whole depth region if the covered depth of irradiation is divided into eight ranges and for each range, the appropriate combination of the first and the second scatterers are used.The method of applying the method to beam with relatively large emittance has been clarified. The six parameters specifying the lateral dose distribution are approximately optimized by changing three free parameters so that those six parameters are near to those which are optimum solution of the zero-emittance case. It is also shown that the dose distribution is rather sensitive to the selection of beam phase space parameters even if the emittance is the same. Namely focused, or diverging, or converging beams will produce different dose distributions.Since the method is sensitive to the beam misalignment, it is necessary to monitor the flatness of the distribution. For that purpose, we manufactured an ionization chamber with four identical collection electrodes placed at four quadrants of a single plane, each of them was electrically isolated from others. We tested the monitor chamber for the pulsed proton beam (5Onsec FWHM) and found that it can be used to monitor the inhomogeneity of less than 1 %.By the research, we clarified that this method can be used for proton cancer therapy effectively.