Beaumont Health System, Royal Oak, Michigan, USA.
J Appl Clin Med Phys. 2022 Apr;23(4):e13543. doi: 10.1002/acm2.13543. Epub 2022 Feb 14.
It has been shown that a widely quoted formula for estimating medical linac photon skyshine equivalent doses is erroneous. Monte Carlo calculations have been performed to develop an easy method for quickly and accurately estimating skyshine radiation levels and to gain improved physical insight into the skyshine phenomenon. Calculations of linac photon skyshine have been performed for 4, 6, 10, 15, and 18 MV beams for 10 × 10 cm and 40 × 40 cm fields and for a range of room dimensions and roof thicknesses. The effect of flattening filter free beams has been considered. Air kerma rates (AKRs) can be accurately fitted to a simple algebraic formula that is a function of the horizontal distance from the isocenter with a single energy dependent fitting parameter. The AKR, at a height of 1.3 m above level ground, reaches a local maximum at a distance d = 1.5d + 1.1h, where d is the horizontal distance from the isocenter to the outside of the side wall, and h is the vertical distance from the isocenter to the top of the roof. For thin roofs, low energy beams lead to significantly more skyshine than high energy beams because low energy photons are more easily scattered through large angles. In the absence of a roof, the maximum skyshine dose rate is on the order of 8 × 10 times the dose rate at isocenter. The average energy of the skyshine photons is about 0.15 MeV, and it is remarkably independent of almost all parameters. A simple methodology is outlined for the evaluation of photon skyshine.
已经表明，一种广泛引用的估算医用直线加速器光子天空散射等效剂量的公式是错误的。通过蒙特卡罗计算，开发了一种快速准确估算天空散射辐射水平的简便方法，并对天空散射现象有了更好的物理理解。对 4、6、10、15 和 18 MV 射线的医用直线加速器光子天空散射进行了计算，采用了 10×10 cm 和 40×40 cm 射野以及一系列房间尺寸和屋顶厚度。还考虑了无均整滤过器的射线束的影响。空气比释动能率（AKR）可以通过一个简单的代数公式进行精确拟合，该公式是一个与水平距离有关的函数，具有一个单一的能量相关拟合参数。在离地面 1.3 米的高度处，AKR 在水平距离等于 1.5d + 1.1h 处达到局部最大值，其中 d 是从等中心点到侧墙外部的水平距离，h 是从等中心点到屋顶顶部的垂直距离。对于薄屋顶，低能射线束比高能射线束导致更多的天空散射，因为低能光子更容易通过大角度散射。在没有屋顶的情况下，天空散射的最大剂量率大约是等中心剂量率的 8×10 倍。天空散射光子的平均能量约为 0.15 MeV，几乎与所有参数都显著无关。概述了一种用于评估光子天空散射的简单方法。
