Iwasaki A
School of Allied Medical Sciences, Hirosaki University, Aomori, Japan.
Med Phys. 1998 Nov;25(11):2209-14. doi: 10.1118/1.598419.
A 10 MV x-ray zero-area TMR (tissue-maximum ratio) expression was constructed using an attenuation coefficient expression which is a function of depth (z) and off-axis distance (r). The zero-area TMR expression can take into account depth and off-axis beam quality change. The construction is based on a method of convolving one-dimension forward and backward primary dose-spread functions with the primary water collision kerma. Additional information required to construct the zero-area TMR expression for a given fanline at an off-axis distance of r is the zero-area TMR value at z = 0 [Ds(r)] and the v(r) and w(r) values in the backward primary dose-spread function. It was found that the depth of peak zero-area TMR [zpeak(r)] decreases with increasing r and that for large depths, the zero-area TMR expression changes significantly as a function of r. Obtaining an accurate expression for zero-area TMR is important for accurate construction of the three-dimensional (3D) forward and backward primary dose-spread functions needed for a 3D convolution method.
利用作为深度（z）和离轴距离（r）函数的衰减系数表达式构建了10 MV X射线零面积组织最大比（TMR）表达式。该零面积TMR表达式可考虑深度和离轴射束质量变化。构建基于将一维正向和反向原发射剂量扩展函数与原发射水碰撞比释动能进行卷积的方法。在离轴距离为r时为给定扇形束构建零面积TMR表达式所需的其他信息是z = 0时的零面积TMR值[Ds(r)]以及反向原发射剂量扩展函数中的v(r)和w(r)值。研究发现，零面积TMR峰值深度[zpeak(r)]随r的增加而减小，并且对于较大深度，零面积TMR表达式随r的变化显著。获得准确的零面积TMR表达式对于精确构建三维（3D）卷积方法所需的三维正向和反向原发射剂量扩展函数很重要。
