Undesirable Radioisotopes Induced by Therapeutic Beams from Medical Linear Accelerators
Undesirable Radioisotopes Induced by Therapeutic Beams from Medical Linear Accelerators
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医用直线加速器的治疗束引起的不良放射性同位素
DOI：
10.5772/22044
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发表时间：
2011
期刊：
影响因子：
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通讯作者：
A. Konefał
中科院分区：
文献类型：
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作者：
A. Konefał
当代的直线加速器通常被称为直线加速器，用于放射医学，产生能量高达20 MeV以上的电子和X射线。这样的能量足以引起产生中子和放射性同位素的核反应。这些中子和放射性同位素在治疗中是不期望的，因为它们是对患者和对操作医用加速器的工作人员的额外剂量的来源。治疗电子和X射线可分别引起电子核（e，e 'n）和光核（e' n，n）反应。这些反应发生在治疗束内的加速器头的大规模组件，主要是在空气中。在X射线的情况下，主要的中子源是束的准直器，平坦化滤波器给出了束的适当轮廓和靶，其中电子被转换为X射线辐射。在电子束的情况下，大多数中子是在准直器系统和散射箔中产生的。这两类反应产生的中子具有宽的能谱，高能端超过10 MeV。大多数中子到达放射治疗设施的混凝土墙壁、天花板和地板。混凝土是一个很好的缓和剂。在这种介质中，中子主要与氢原子核发生弹性碰撞。减速的中子可能会离开混凝土并返回到空气中，从而有助于放射治疗设施内中子能量的特定分布。减速中子的动能分布符合麦克斯韦-玻尔兹曼分布定律。中子在热能和共振能范围内很容易引起简单的俘获（n，n）反应，产生放射性同位素。整个加速器室内的中子场几乎是均匀的。因此，源自中子反应的放射性同位素可以在加速器部件和配件以及放射治疗设施的墙壁、天花板和地板中产生。此外，中子可以在放射治疗设施的入口门中引起简单的俘获反应。这些中子反应的结果是放射出穿透性伽马射线。因此，在发射高能治疗射束期间，伽马辐射可能出现在操作室中的放射治疗设施门附近。本文对放射治疗设备的加速器部件、附件、墙壁、天花板、地板和门中的放射性同位素以及
Contemporary linear accelerators called often linacs, used in radiation medicine generate electrons and X-rays with energies up to over 20 MeV. Such energies are enough to induce nuclear reactions in which neutrons and radioisotopes are produced. These neutrons and radioisotopes are undesirable in therapy, because they are source of an additional dose to patients and to staff operating the medical accelerators. The therapeutic electrons and X-rays can induce electronuclear (e,e’n) and photonuclear (,n) reactions, respectively. These reactions take place inside the therapeutic beam in massive components of an accelerator head, mainly and in air. In the case of the X-rays the main neutron sources are the collimators of the beam, flattening filter giving the appropriate profile of the beam and the target in which electrons are converted into X-ray radiation. In the case of the electron beams the majority of neutrons are produced in the collimator system and in the scattered foils. The neutrons originated in both mentioned type of reactions have the broad energy spectrum with the high-energy end of more than ten MeV. Majority of the neutrons reach the concrete walls, ceiling and floor of the radiotherapy facility. Concrete is a good moderator. In this medium the neutrons undergo elastic collisions with nuclei of hydrogen, mainly. The slowed down neutrons may get out of concrete and return to air, contributing to the specific distribution of neutron energy inside the radiotherapy facility. Kinetic energies of the slowed down neutrons are distributed according to the Maxwell-Boltzmann distribution law. The neutrons can easy induce the simple capture (n,) reactions in the thermal and resonance energy range and radioisotopes are produced. The neutron field is almost uniform in whole accelerator room. Thus the radioisotopes originating from the neutron reactions can be produced in the accelerator components and accessories as well as in the wall, ceiling and floor of the radiotherapy facility. Moreover, the neutrons can induce simple capture reactions in the entrance door of the radiotherapy facility. The penetrative gammas are emitted as a result of these neutron reactions. Therefore, the gamma radiation can appear close to the radiotherapy facility door in the operator room during emission the high-energy therapeutic beams. In the paper the radioisotopes originating in the accelerator components and in the accessories as well as in the walls, ceiling, floor and door of the radiotherapy facility and in