Yonai Shunsuke, Matsufuji Naruhiro, Namba Masao
National Institute of Radiological Sciences 4-9-1, Anagawa, Inage-ku, Chiba, Chiba 263-8555, Japan.
Med Phys. 2012 Aug;39(8):5028-39. doi: 10.1118/1.4736823.
Recent radiotherapy technologies including carbon-ion radiotherapy can improve the dose concentration in the target volume, thereby not only reducing side effects in organs at risk but also the secondary cancer risk within or near the irradiation field. However, secondary cancer risk in the low-dose region is considered to be non-negligible, especially for younger patients. To achieve a dose estimation of the whole body of each patient receiving carbon-ion radiotherapy, which is essential for risk assessment and epidemiological studies, Monte Carlo simulation plays an important role because the treatment planning system can provide dose distribution only in∕near the irradiation field and the measured data are limited. However, validation of Monte Carlo simulations is necessary. The primary purpose of this study was to establish a calculation method using the Monte Carlo code to estimate the dose and quality factor in the body and to validate the proposed method by comparison with experimental data. Furthermore, we show the distributions of dose equivalent in a phantom and identify the partial contribution of each radiation type.
We proposed a calculation method based on a Monte Carlo simulation using the PHITS code to estimate absorbed dose, dose equivalent, and dose-averaged quality factor by using the Q(L)-L relationship based on the ICRP 60 recommendation. The values obtained by this method in modeling the passive beam line at the Heavy-Ion Medical Accelerator in Chiba were compared with our previously measured data.
It was shown that our calculation model can estimate the measured value within a factor of 2, which included not only the uncertainty of this calculation method but also those regarding the assumptions of the geometrical modeling and the PHITS code. Also, we showed the differences in the doses and the partial contributions of each radiation type between passive and active carbon-ion beams using this calculation method. These results indicated that it is essentially important to include the dose by secondary neutrons in the assessment of the secondary cancer risk of patients receiving carbon-ion radiotherapy with active as well as passive beams.
We established a calculation method with a Monte Carlo simulation to estimate the distribution of dose equivalent in the body as a first step toward routine risk assessment and an epidemiological study of carbon-ion radiotherapy at NIRS. This method has the advantage of being verifiable by the measurement.
包括碳离子放疗在内的近期放疗技术能够提高靶区内的剂量集中度，从而不仅减少危及器官的副作用，还能降低照射野内或其附近的二次癌症风险。然而，低剂量区域的二次癌症风险被认为不可忽视，尤其是对于年轻患者。为了实现对接受碳离子放疗的每位患者全身剂量的估计，这对于风险评估和流行病学研究至关重要，蒙特卡罗模拟发挥着重要作用，因为治疗计划系统只能提供照射野内/附近的剂量分布，且实测数据有限。然而，蒙特卡罗模拟的验证是必要的。本研究的主要目的是建立一种使用蒙特卡罗代码来估计体内剂量和品质因数的计算方法，并通过与实验数据比较来验证所提出的方法。此外，我们展示了模体中的剂量当量分布，并确定了每种辐射类型的部分贡献。
我们提出了一种基于使用PHITS代码的蒙特卡罗模拟的计算方法，通过基于国际辐射防护委员会（ICRP）60号建议的Q(L)-L关系来估计吸收剂量、剂量当量和剂量平均品质因数。将该方法在模拟千叶重离子医学加速器的被动束流线路时获得的值与我们之前的实测数据进行比较。
结果表明，我们的计算模型能够在2倍因子范围内估计实测值，这不仅包括该计算方法的不确定性，还包括几何建模假设和PHITS代码的不确定性。此外，我们使用该计算方法展示了被动和主动碳离子束之间剂量的差异以及每种辐射类型的部分贡献。这些结果表明，在评估接受主动和被动束流碳离子放疗患者的二次癌症风险时，纳入次级中子的剂量至关重要。
我们建立了一种蒙特卡罗模拟计算方法，以估计体内剂量当量分布，这是迈向近畿大学重离子科学研究所碳离子放疗常规风险评估和流行病学研究的第一步。该方法具有可通过测量进行验证的优点。
