Inaniwa Taku, Tomitani Takehiro, Kohno Toshiyuki, Kanai Tatsuaki
Department of Energy Sciences, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan.
Phys Med Biol. 2005 Mar 21;50(6):1131-45. doi: 10.1088/0031-9155/50/6/007. Epub 2005 Feb 23.
In radiation therapy with hadron beams, it is important to evaluate the range of incident ions and the deposited dose distribution in a patient body for the effective utilization of such properties as the dose concentration and the biological effect around the Bragg peak. However, there is some ambiguity in determining this range because of a conversion error from the x-ray CT number to the charged particle range. This is because the CT number is related to x-ray absorption coefficients, while the ion range is determined by the electron density of the substance. Using positron emitters produced in the patient body through fragmentation reactions during the irradiation has been proposed to overcome this problem. The activity distribution in the patient body can be deduced by detecting pairs of annihilation gamma rays emitted from the positron emitters, and information about the range of incident ions can be obtained. In this paper, we propose a quantitative comparison method to evaluate the mean range of incident ions and monitor the activity distribution related to the deposited dose distribution. The effectiveness of the method was demonstrated by evaluating the range of incident ions using the maximum likelihood estimation (MLE) method and Fisher's information was calculated under realistic conditions for irradiations with several kinds of ions. From the calculated Fisher's information, we compared the relative advantages of initial beams to determine the range of incident ions. The (16)O irradiation gave the most information among the stable heavy ions when we measured the induced activity for 500 s and 60 s just after the irradiation. Therefore, under these conditions, we concluded that the (16)O beam was the optimum beam to monitor the activity distribution and to evaluate the range. On the other hand, if the positron emitters were injected directly as a therapeutic beam, the (15)O irradiation gave the most information. Although the relative advantages of initial beams as well as the measured activity distributions slightly varied according to the measurement conditions, comparisons could be made for different conditions by using Fisher's information.
在强子束放射治疗中，为有效利用布拉格峰周围的剂量集中和生物效应等特性，评估入射离子的射程以及患者体内的剂量沉积分布非常重要。然而，由于从X射线CT值到带电粒子射程的转换误差，在确定该射程时存在一定的模糊性。这是因为CT值与X射线吸收系数相关，而离子射程由物质的电子密度决定。有人提出利用辐照过程中通过碎裂反应在患者体内产生的正电子发射体来克服这个问题。通过检测正电子发射体发射的湮灭伽马射线对，可以推断患者体内的活度分布，并获得有关入射离子射程的信息。在本文中，我们提出了一种定量比较方法，用于评估入射离子的平均射程并监测与剂量沉积分布相关的活度分布。通过使用最大似然估计（MLE）方法评估入射离子的射程，并在几种离子辐照的实际条件下计算费舍尔信息，证明了该方法的有效性。根据计算出的费舍尔信息，我们比较了初始束在确定入射离子射程方面的相对优势。当我们在辐照后立即测量500秒和60秒的诱导活度时，（16）O辐照在稳定重离子中提供的信息最多。因此，在这些条件下，我们得出结论，（16）O束是监测活度分布和评估射程的最佳束。另一方面，如果将正电子发射体直接作为治疗束注入，则（15）O辐照提供的信息最多。尽管初始束的相对优势以及测量的活度分布会根据测量条件略有变化，但通过使用费舍尔信息可以对不同条件进行比较。
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