Martínez-Ovalle S A, Barquero R, Gómez-Ros J M, Lallena A M
Grupo de Física Nuclear Aplicada y Simulación, Universidad Pedagógica y Tecnológica de Colombia, Tunja, Colombia.
Radiat Prot Dosimetry. 2012 Mar;148(4):457-64. doi: 10.1093/rpd/ncr208. Epub 2011 Jul 12.
In this work, the ambient dose equivalent, H*(10), due to neutrons outside three bunkers that house a 15- and a 18-MV Varian Clinac 2100C/D and a 15-MV Elekta Inor clinical linacs, has been calculated. The Monte Carlo code MCNPX (v. 2.5) has been used to simulate the neutron production and transport. The complete geometries including linacs and full installations have been built up according to the specifications of the manufacturers and the planes provided by the corresponding medical physical services of the hospitals where the three linacs operate. Two of these installations, those lodging the Varian linacs, have an entrance door to the bunker while the other one does not, although it has a maze with two bends. Various treatment orientations were simulated in order to establish plausible annual equivalent doses. Specifically anterior-posterior, posterior-anterior, left lateral, right lateral orientations and an additional one with the gantry rotated 30° have been studied. Significant dose rates have been found only behind the walls and the door of the bunker, near the entrance and the console, with a maximum of 12 µSv h(-1). Dose rates per year have been calculated assuming a conservative workload for the three facilities. The higher dose rates in the corresponding control areas were 799 µSv y(-1), in the case of the facility which operates the 15-MV Clinac, 159 µSv y(-1), for that with the 15-MV Elekta, and 21 µSv y(-1) for the facility housing the 18-MV Varian. A comparison with measurements performed in similar installations has been carried out and a reasonable agreement has been found. The results obtained indicate that the neutron contamination does not increase the doses above the legal limits and does not produce a significant enhancement of the dose equivalent calculated. When doses are below the detection limits provided by the measuring devices available today, MCNPX simulation provides an useful method to evaluate neutron dose equivalents based on a detailed description of linac, patient and bunker.
在这项工作中，已计算了容纳一台15兆伏和一台18兆伏瓦里安Clinac 2100C/D直线加速器以及一台15兆伏医科达Inor临床直线加速器的三个掩体外部因中子产生的环境剂量当量H*(10)。已使用蒙特卡罗代码MCNPX（版本2.5）来模拟中子的产生和输运。已根据制造商的规格以及三台直线加速器所在医院相应医学物理服务部门提供的平面图，构建了包括直线加速器和完整设施的完整几何模型。其中两个设施，即安置瓦里安直线加速器的设施，掩体有一个入口门，而另一个没有入口门，不过它有一个带有两个转弯的迷宫通道。模拟了各种治疗方向，以确定合理的年当量剂量。具体而言，研究了前后、后前、左侧、右侧方向以及一种龙门架旋转30°的额外方向。仅在掩体的墙壁和门后、入口和控制台附近发现了显著的剂量率，最大值为12微希沃特每小时（μSv h(-1)）。假设这三个设施的工作量较为保守，计算了每年的剂量率。对于运行15兆伏直线加速器的设施，相应控制区域的较高剂量率为799微希沃特每年（μSv y(-1)）；对于配备15兆伏医科达直线加速器的设施，为159微希沃特每年；对于容纳18兆伏瓦里安直线加速器的设施，为21微希沃特每年。已与在类似设施中进行的测量进行了比较，并发现了合理的一致性。所获得的结果表明，中子污染不会使剂量增加到法定限值以上，也不会导致计算出的剂量当量显著增加。当剂量低于当今可用测量设备提供的检测限时，MCNPX模拟提供了一种基于对直线加速器、患者和掩体的详细描述来评估中子剂量当量的有用方法。
