National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan.
Med Phys. 2009 Oct;36(10):4830-9. doi: 10.1118/1.3220624.
Recent successful results in passive carbon-ion radiotherapy allow the patient to live for a longer time and allow younger patients to receive the radiotherapy. Undesired radiation exposure in normal tissues far from the target volume is considerably lower than that close to the treatment target, but it is considered to be non-negligible in the estimation of the secondary cancer risk. Therefore, it is very important to reduce the undesired secondary neutron exposure in passive carbon-ion radiotherapy without influencing the clinical beam. In this study, the source components in which the secondary neutrons are produced during passive carbon-ion radiotherapy were identified and the method to reduce the secondary neutron dose effectively based on the identification of the main sources without influencing the clinical beam was investigated.
A Monte Carlo study with the PHITS code was performed by assuming the beamline at the Heavy-Ion Medical Accelerator in Chiba (HIMAC). At first, the authors investigated the main sources of secondary neutrons in passive carbon-ion radiotherapy. Next, they investigated the reduction in the neutron dose with various modifications of the beamline device that is the most dominant in the neutron production. Finally, they investigated the use of an additional shield for the patient.
It was shown that the main source is the secondary neutrons produced in the four-leaf collimator (FLC) used as a precollimator at HIAMC, of which contribution in the total neutron ambient dose equivalent is more than 70%. The investigations showed that the modification of the FLC can reduce the neutron dose at positions close to the beam axis by 70% and the FLC is very useful not only for the collimation of the primary beam but also the reduction in the secondary neutrons. Also, an additional shield for the patient is very effective to reduce the neutron dose at positions farther than 50 cm from the beam axis. Finally, they showed that the neutron dose can be reduced by approximately 70% at any position without influencing the primary beam used in treatment.
This study was performed by assuming the HIMAC beamline; however, this study provides important information for reoptimizing the arrangement and the materials of beamline devices and designing a new facility for passive carbon-ion radiotherapy and probably passive proton radiotherapy.
最近被动碳离子放射治疗的成功结果使患者能够活得更久，并且允许年轻患者接受放射治疗。与治疗靶区接近的组织相比，远离靶区的正常组织的不期望辐射暴露要低得多，但在估计二次癌症风险时，被认为是不可忽视的。因此，在不影响临床射束的情况下，降低被动碳离子放射治疗中不期望的二次中子照射非常重要。在这项研究中，确定了在被动碳离子放射治疗过程中产生二次中子的源组件，并研究了基于主要源的识别而不影响临床射束的有效降低二次中子剂量的方法。
通过假设在千叶重离子医疗加速器（HIMAC）的射束线进行了蒙特卡罗研究与 PHITS 代码。首先，作者研究了被动碳离子放射治疗中二次中子的主要来源。接下来，他们研究了通过改变在中子产生中最占主导地位的射束线设备来降低中子剂量。最后，他们研究了为患者使用附加屏蔽的方法。
结果表明，主要来源是作为 HIAMC 预准直器使用的四叶准直器（FLC）中产生的二次中子，其在总中子环境剂量当量中的贡献超过 70％。研究表明，FLC 的修改可以将靠近射束轴的位置的中子剂量降低 70％，并且 FLC 不仅对初级射束的准直有用，而且对降低二次中子也有用。此外，患者的附加屏蔽对于降低远离射束轴 50cm 以外的位置的中子剂量非常有效。最后，他们表明，在不影响用于治疗的初级射束的情况下，可以在任何位置将中子剂量降低约 70％。
本研究是在假设 HIMAC 射束线的情况下进行的；但是，这项研究为重新优化射束线设备的布置和材料以及设计被动碳离子放射治疗和可能的被动质子放射治疗的新设施提供了重要信息。
