Rustgi S N, Rustgi A K, Jiang S B, Ayyangar K M
Department of Radiology, MetroHealth Medical Center/Case Western Reserve University, Cleveland, OH 44109, USA.
Phys Med Biol. 1998 Dec;43(12):3509-18. doi: 10.1088/0031-9155/43/12/009.
The influence of high-density tissue heterogeneities in small-diameter beams used in stereotactic radiosurgery has been investigated. Dose perturbation immediately behind aluminium sheets, used to simulate a high-density tissue inhomogeneity such as bone, was studied in a solid water phantom. Dose reduction factors (DRFs), which are the ratios of the dose in the presence of the inhomogeneity to dose in a uniform density solid water phantom, were measured with a diamond detector for three thicknesses of aluminium. DRFs exhibit dependence on both the inhomogeneity thickness and the beam diameter. The DRF decreases with inhomogeneity thickness. The DRF initially decreases with increase in the beam diameter from 12.5 to 25 mm. For fields greater than 25 mm, the DRFs are nearly constant. The commonly used algorithms such as the TAR ratio method underestimate the magnitude of the measured effect. A good agreement between these measurements and Monte Carlo calculations is obtained. The influence of the high-density inhomogeneity on the tissue maximum ratio (TMR) was also measured with the inhomogeneity at a fixed depth dmax from the entrance surface. The TMR is reduced for all detector-inhomogeneity distances investigated. The dose build-up phenomenon observed in the presence of low-density air inhomogeneity is absent in the presence of a high-density inhomogeneity. The beam width (defined by 50% dose points) immediately beyond the inhomogeneity is unaffected by the high-density inhomogeneity. However, the 90%-10% and 80%-20% dose penumbra widths and the dose outside the beam edge (beyond the 50% dose point) are reduced. This reduction in dose outside the beam edge is caused by the reduced range of the secondary radiation (photons and electrons) in the high-density medium.
立体定向放射治疗中使用的小直径射束内高密度组织不均匀性的影响已得到研究。在固体水模体中研究了铝板后方紧邻处的剂量扰动，铝板用于模拟诸如骨骼等高密度组织不均匀性。用金刚石探测器对三种厚度的铝板测量了剂量降低因子（DRF），其为存在不均匀性时的剂量与均匀密度固体水模体中剂量的比值。DRF既取决于不均匀性厚度，也取决于射束直径。DRF随不均匀性厚度减小。DRF最初随射束直径从12.5毫米增加到25毫米而减小。对于大于25毫米的射野，DRF几乎恒定。常用算法如组织空气比（TAR）比值法低估了测量效应的大小。这些测量结果与蒙特卡罗计算结果取得了良好的一致性。还在距入射面固定深度dmax处设置不均匀性，测量了高密度不均匀性对组织最大剂量比（TMR）的影响。在所研究的所有探测器与不均匀性距离处，TMR均降低。在存在低密度空气不均匀性时观察到的剂量积累现象在存在高密度不均匀性时不存在。紧邻不均匀性后方的射束宽度（由50%剂量点定义）不受高密度不均匀性影响。然而，90%-10%和80%-20%剂量半值层宽度以及射束边缘外（超过50%剂量点）的剂量减小。射束边缘外剂量的这种减小是由高密度介质中二次辐射（光子和电子）射程减小所致。
