Taylor P L, Rodgers J E, Shobe J
Department of Radiation Medicine, Georgetown University Medical Center, Washington, DC 20007, USA.
Med Phys. 1999 Aug;26(8):1442-6. doi: 10.1118/1.598670.
Computation of shielding requirements for a linear accelerator must take into account the amount of radiation scattered from the patient to areas outside the primary beam. Currently, the most frequently used data are from NCRP 49 that only includes data for x-ray energies up to 6 MV and angles from 30 degrees to 135 degrees. In this work we have determined by Monte Carlo simulation the scattered fractions of dose for a wide range of energies and angles of clinical significance including 6, 10, 18, and 24 MV and scattering angles from 10 degrees to 150 degrees. Calculations were made for a 400 cm2 circular field size impinging onto a spherical phantom. Scattered fractions of dose were determined at 1 m from the phantom. Angles from 10 degrees to 30 degrees are of concern for higher energies where the scatter is primarily in the forward direction. An error in scatter fraction may result in too little secondary shielding near the junction with the primary barrier. The Monte Carlo code ITS (Version 3.0) developed at Sandia National Laboratory and NIST was used to simulate scatter from the patient to the barrier. Of significance was the variation of calculated scattered dose with depth of measurement within the barrier indicating that accurate values may be difficult to obtain. Mean energies of scatter x-ray spectra are presented.
直线加速器屏蔽要求的计算必须考虑从患者散射到原射线束以外区域的辐射量。目前，最常用的数据来自NCRP 49，该报告仅包含能量高达6兆伏、角度在30度至135度范围内的X射线数据。在这项工作中，我们通过蒙特卡罗模拟确定了一系列具有临床意义的能量和角度（包括6、10、18和24兆伏以及10度至150度的散射角）下的剂量散射分数。计算是针对照射到球形体模上的400平方厘米圆形野尺寸进行的。在距体模1米处确定剂量散射分数。对于较高能量，10度至30度的角度是需要关注的，因为此时散射主要在前向方向。散射分数的误差可能导致在与主防护层交界处附近的二次屏蔽过少。使用桑迪亚国家实验室和美国国家标准与技术研究院开发的蒙特卡罗代码ITS（版本3.0）来模拟从患者到防护层的散射。值得注意的是，计算得到的散射剂量随防护层内测量深度的变化，这表明可能难以获得准确值。文中给出了散射X射线谱的平均能量。
