Mohammadi N, Miri-Hakimabad S H, Rafat-Motavalli L
Physics Department, Faculty of Sciences, Ferdowsi University of Mashhad, Iran.
J Biomed Phys Eng. 2014 Dec 15;4(4):127-40. eCollection 2014 Dec.
High-energy linear accelerator (linac) is a valuable tool and the most commonly used device for external beam radiation treatments in cancer patients. In the linac head, high-energy photons with energies above the threshold of (γ,n) interaction produce photoneutrons. These photoneutrons deliver the extra dose to the patients undergoing radiation treatment and increase the risk of secondary cancer.
In this study, a simplified model of the linac head was simulated and photoneutron dose equivalent was calculated at the isocenter and maze in the sphere detector. In addition, the absorbed and equivalent dose of photoneutron were estimated in the some organs of the phantom.
The simulations were made using the Monte Carlo code. The ICRP reference adult male voxel phantom was used as the human body model for dosimetry calculations.
The results of dose calculations at the isocenter and maze showed that photoneutron dose decreases as the function of distance from the isocenter and increases with increasing the distance from the entrance maze.
It is concluded that the simplified model of linac head is a useful and reliable method in dosimetry calculations. Calculations illustrated that the photoneutron dose is not negligible and duo to its harmful biological effects on body, it should be considered in the treatment plans.
高能直线加速器（直线加速器）是一种有价值的工具，也是癌症患者外照射放疗中最常用的设备。在直线加速器机头中，能量高于（γ，n）相互作用阈值的高能光子会产生光中子。这些光中子会给接受放射治疗的患者带来额外剂量，并增加患继发性癌症的风险。
在本研究中，对直线加速器机头的简化模型进行了模拟，并计算了球形探测器等中心和迷宫处的光中子剂量当量。此外，还估算了体模某些器官中的光中子吸收剂量和当量剂量。
使用蒙特卡罗代码进行模拟。国际辐射防护委员会（ICRP）参考成年男性体素体模用作剂量学计算的人体模型。
等中心和迷宫处的剂量计算结果表明，光中子剂量随与等中心距离的增加而降低，随与迷宫入口距离的增加而增加。
得出结论，直线加速器机头的简化模型是剂量学计算中一种有用且可靠的方法。计算表明，光中子剂量不可忽略，由于其对身体有害的生物学效应，在治疗计划中应予以考虑。
