Physics Department, University of Novi Sad, Novi Sad, Serbia.
Med Phys. 2012 Mar;39(3):1278-81. doi: 10.1118/1.3682313.
This study was undertaken to explore the effects of the jaws and the MLC openings on the neutron dose equivalent (DE) at the maze door and neutron flux at the patient plane.
The neutron dose equivalent was measured at the maze entrance door of a 15 MV therapy linear accelerator room. All measurements were performed using various field sizes up to 40 cm × 40 cm. Activation detectors constructed from natural Indium (In) were exposed at Cd envelope to neutrons in order to estimate relative changes of epithermal neutron fluences in the patient plane.
Our study showed that the dose equivalent at the maze door is at the highest when the jaw are closed and that maximal jaws opening reduces the DE by more than 20%. The neutron dose equivalent at the maze door measured for radiation fields defined by jaws do not differ significantly from the DE measured when MLC determines the same size radiation field. The epithermal capture reaction rate measured using different jaw openings differs by approximately 10%. When an MLC leaf is inserted into a fixed geometry for one opening of the jaws, an increase of the epithermal neutron capture reaction rate in Indium activation detectors was observed.
There is no significant difference in the neutron DE when MLC defines radiation field instead of jaws. This leads to the conclusion that the overall number of neutrons remains similar and it does not depend on how primary photon beam was stopped-by the jaws or the MLC. An increase of the fast neutron capture reaction rate when MLC leaves are inserted probably originates from the neutron scattering.
本研究旨在探讨在迷宫门处的中子剂量当量（DE）和患者平面处的中子通量受颌骨和多叶准直器（MLC）开口的影响。
在 15MV 治疗直线加速器机房的迷宫入口门处测量了中子剂量当量。所有测量均使用各种大小的射野，最大可达 40cm×40cm。使用天然铟（In）制成的激活探测器在 Cd 包壳中暴露于中子，以估计患者平面中热中子通量的相对变化。
我们的研究表明，当颌骨关闭时，迷宫门处的剂量当量最高，最大颌骨开口可使 DE 降低 20%以上。由颌骨定义的辐射场测量的迷宫门处的中子剂量当量与 MLC 确定相同大小的辐射场时测量的 DE 没有显著差异。使用不同的颌骨开口测量的热中子俘获反应率相差约 10%。当 MLC 叶片插入固定的颌骨开口几何形状时，观察到铟激活探测器中热中子俘获反应率增加。
当 MLC 定义辐射场而不是颌骨时，中子 DE 没有显著差异。这得出结论，总的中子数量相似，而与初级光子束是由颌骨还是 MLC 阻止无关。当 MLC 叶片插入时，快中子俘获反应率的增加可能源自中子散射。
