Ahuja S D, Stroup S L, Bolin M G, Gibbs S J
Med Phys. 1980 Mar-Apr;7(2):120-6. doi: 10.1118/1.594674.
The physical basis of deposition of radiation dose within a homogeneous phantom irradiated by a monoenergetic photon beam has been studied in terms of photon attenuation and energy-absorption properties of the phantom material. A semi-empirical model based on the Klein-Nishina formula for Compton scattering, and the ratio of multiply scattered to singly scattered photon fluences, has been developed for the scatter dose component within a realistic phantom to determine the central-axial percent depth dose (PDD) and off-central-axis ratios (OCR). Differences between the predicted and measured values of PDD and OCR for cobalt-60 and cesium-137 beams are less than 3% for fields of equivalent-square-side less than 20 cm, and less than 5% for larger fields. Beam profiles of all field sizes can be well simulated by this model and reasonable agreement has been found between the predicted and tabulated values of scatter functions and the backsetter factor for cobalt-60 beams. This formulation involves no variable parameters, and is valid for all values of the source-to-surface distance, field length and width, and field shape. However, the algorithm developed is not suitable for routine multiple-field treatment planning because it requires large computer memory size.
已根据体模材料的光子衰减和能量吸收特性，研究了单能光子束照射均匀体模时辐射剂量沉积的物理基础。基于康普顿散射的克莱因-仁科公式以及多次散射光子注量与单次散射光子注量之比，开发了一个半经验模型，用于确定实际体模内的散射剂量分量，以计算中心轴百分深度剂量（PDD）和离轴比（OCR）。对于钴-60和铯-137射线束，当等效方边边长小于20 cm时，PDD和OCR的预测值与测量值之间的差异小于3%；对于较大射野，差异小于5%。该模型可以很好地模拟所有射野尺寸的射野轮廓，并且发现钴-60射线束的散射函数和后置因子的预测值与表格值之间具有合理的一致性。该公式不涉及可变参数，并且对于源皮距、射野长度和宽度以及射野形状的所有值均有效。然而，所开发的算法不适用于常规多野治疗计划，因为它需要较大的计算机内存。
