Nizin P, Qian G X, Rashid H
Baylor College of Medicine, Department of Radiology, Houston, Texas 77030-3498.
Med Phys. 1993 Sep-Oct;20(5):1353-60. doi: 10.1118/1.597155.
A procedure of separating the primary- and scatter-dose components in therapeutic photon beams is examined. It is based on the observation that the scatter-dose component is proportional to the variable z = rd/(r+d), where r and d are beam radius and depth in phantom, respectively. It is, therefore, possible to express an absorbed dose in the form of a linear equation D(r) = P+Nz, where at a fixed depth d, both primary dose P and coefficient N are constant. A method of linear extrapolation of an absorbed dose D(r) to "zero-field" size, i.e., r = 0, is utilized. Since Monte Carlo technique is capable of scoring separately the primary- and scatter-dose components, it is used to evaluate the accuracy of the linear extrapolation method within the range of 60Co-15-MV nominal photon energies. The results demonstrate that this method is sufficiently accurate to obtain the primary dose component in photon beams. For 60Co gamma radiation in water, tabulated sets of measured depth-dose data are analyzed by the linear extrapolation method to review "zero-field" dose values [percentage depth dose (PDD) and tissue-air ratio (TAR) tables of the British Journal of Radiology, Suppl. 17]. The "zero-field" PDD data are found to be accurate within limits of experimental uncertainties. Inconsistencies in the TAR table are illustrated and discussed. 60Co tables of relative doses, D(r,d)/P(dmax), including "zero-field" values for both fixed SSD and isocentric geometries, are generated. Dose calculation in irregular fields is considered. The linear extrapolation method is recommended as a standard procedure for separating primary dose from depth-dose data in high-energy photon beams.
本文研究了一种分离治疗性光子束中主剂量和散射剂量成分的方法。该方法基于这样的观察结果：散射剂量成分与变量z = rd/(r + d)成正比，其中r和d分别为射束半径和模体中的深度。因此，吸收剂量可以用线性方程D(r) = P + Nz的形式表示，在固定深度d处，主剂量P和系数N均为常数。利用了一种将吸收剂量D(r)线性外推至“零射野”尺寸（即r = 0）的方法。由于蒙特卡罗技术能够分别计算主剂量和散射剂量成分，因此用它来评估在60Co - 15 MV标称光子能量范围内线性外推法的准确性。结果表明，该方法足够准确，能够获得光子束中的主剂量成分。对于水中的60Co伽马射线，通过线性外推法分析了测量的深度剂量数据列表，以复查“零射野”剂量值[《英国放射学杂志》增刊17中的百分深度剂量（PDD）和组织空气比（TAR）表]。发现“零射野”PDD数据在实验不确定度范围内是准确的。说明了并讨论了TAR表中的不一致性。生成了60Co相对剂量表D(r, d)/P(dmax)，包括固定源皮距和等中心几何条件下的“零射野”值。考虑了不规则射野中的剂量计算。推荐将线性外推法作为从高能光子束深度剂量数据中分离主剂量的标准程序。
