Nedaie Hassan Ali, Darestani Hoda, Banaee Nooshin, Shagholi Negin, Mohammadi Kheirollah, Shahvar Arjang, Bayat Esmaeel
Departement of Radiotherapy Oncology, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran.
Department of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.
J Med Phys. 2014 Jan;39(1):10-7. doi: 10.4103/0971-6203.125476.
High-energy linacs produce secondary particles such as neutrons (photoneutron production). The neutrons have the important role during treatment with high energy photons in terms of protection and dose escalation. In this work, neutron dose equivalents of 18 MV Varian and Elekta accelerators are measured by thermoluminescent dosimeter (TLD) 600 and TLD700 detectors and compared with the Monte Carlo calculations. For neutron and photon dose discrimination, first TLDs were calibrated separately by gamma and neutron doses. Gamma calibration was carried out in two procedures; by standard 60Co source and by 18 MV linac photon beam. For neutron calibration by (241)Am-Be source, irradiations were performed in several different time intervals. The Varian and Elekta linac heads and the phantom were simulated by the MCNPX code (v. 2.5). Neutron dose equivalent was calculated in the central axis, on the phantom surface and depths of 1, 2, 3.3, 4, 5, and 6 cm. The maximum photoneutron dose equivalents which calculated by the MCNPX code were 7.06 and 2.37 mSv.Gy(-1) for Varian and Elekta accelerators, respectively, in comparison with 50 and 44 mSv.Gy(-1) achieved by TLDs. All the results showed more photoneutron production in Varian accelerator compared to Elekta. According to the results, it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry inside the linac field due to high photon flux, while MCNPX code is an appropriate alternative for studying photoneutron production.
高能直线加速器会产生诸如中子等次级粒子（光中子产生）。在高能光子治疗过程中，中子在防护和剂量提升方面具有重要作用。在这项工作中，使用热释光剂量计（TLD）600和TLD700探测器测量了18 MV瓦里安和医科达加速器的中子剂量当量，并与蒙特卡罗计算结果进行了比较。为了区分中子和光子剂量，首先分别用伽马和中子剂量对TLD进行校准。伽马校准分两个步骤进行；通过标准60Co源和18 MV直线加速器光子束。对于用（241）Am-Be源进行的中子校准，在几个不同的时间间隔进行辐照。用MCNPX代码（版本2.5）模拟了瓦里安和医科达直线加速器机头及模体。在模体中心轴、表面以及1、2、3.3、4、5和6 cm深度处计算中子剂量当量。MCNPX代码计算得出的瓦里安和医科达加速器的最大光中子剂量当量分别为7.06和2.37 mSv·Gy-1，而TLD测得的分别为50和44 mSv·Gy-1。所有结果表明，与医科达加速器相比，瓦里安加速器产生的光中子更多。根据结果，由于光子通量高，TLD600和TLD700对似乎不是直线加速器射野内中子剂量测定的合适剂量计，而MCNPX代码是研究光中子产生的合适替代方法。
