Medical Physics Department, Lower Silesian Oncology Center, Hiszfeld Square 12, 53-413, Wroclaw, Poland,
Strahlenther Onkol. 2014 May;190(5):459-66. doi: 10.1007/s00066-014-0635-z. Epub 2014 Mar 6.
The goal of this work was to assess the additional dose from secondary neutrons and γ-rays generated during total body irradiation (TBI) using a medical linac X-ray beam.
Nuclear reactions that occur in the accelerator construction during emission of high-energy beams in teleradiotherapy are the source of secondary radiation. Induced activity is dependent on the half-lives of the generated radionuclides, whereas neutron flux accompanies the treatment process only.
The TBI procedure using a 18 MV beam (Clinac 2100) was considered. Lateral and anterior-posterior/posterior-anterior fractions were investigated during delivery of 2 Gy of therapeutic dose. Neutron and photon flux densities were measured using neutron activation analysis (NAA) and semiconductor spectrometry. The secondary dose was estimated applying the fluence-to-dose conversion coefficients.
The main contribution to the secondary dose is associated with fast neutrons. The main sources of γ-radiation are the following: (56)Mn in the stainless steel and (187)W of the collimation system as well as positron emitters, activated via (n,γ) and (γ,n) processes, respectively. In addition to 12 Gy of therapeutic dose, the patient could receive 57.43 mSv in the studied conditions, including 4.63 μSv from activated radionuclides.
Neutron dose is mainly influenced by the time of beam emission. However, it is moderated by long source-surface distances (SSD) and application of plexiglass plates covering the patient body during treatment. Secondary radiation gives the whole body a dose, which should be taken into consideration especially when one fraction of irradiation does not cover the whole body at once.
本研究旨在评估全身放疗（TBI）过程中，使用医用直线加速器 X 射线束产生的次级中子和γ射线的附加剂量。
在远程放射治疗中发射高能束时，在加速器结构中发生的核反应是次级辐射的来源。诱发活性取决于生成放射性核素的半衰期，而中子通量仅伴随治疗过程。
考虑使用 18 MV 束（Clinac 2100）进行 TBI 程序。在给予 2 Gy 治疗剂量时，研究了侧向和前后/后前部分。使用中子活化分析（NAA）和半导体谱仪测量中子和光子通量密度。应用注量-剂量转换系数估算次级剂量。
次级剂量的主要贡献与快中子有关。γ 辐射的主要来源如下：不锈钢中的（56）Mn 和准直系统中的（187）W 以及通过（n，γ）和（γ，n）过程分别激活的正电子发射体。除了 12 Gy 的治疗剂量外，在研究条件下，患者还可能接受 57.43 mSv 的剂量，其中包括 4.63 μSv 来自活化的放射性核素。
中子剂量主要受束发射时间的影响。然而，它可以通过长源-表面距离（SSD）和在治疗过程中应用覆盖患者身体的有机玻璃板来调节。次级辐射会对全身产生剂量，特别是当一次照射的剂量不能立即覆盖全身时，应特别考虑。
