Webb S, Fox R A
Phys Med Biol. 1980 Mar;25(2):225-40. doi: 10.1088/0031-9155/25/2/003.
A Monte Carlo computer program has been used to calculate axial and off-axis depth dose distributions arising from the interaction of an external beam of 60Co radiation with a medium containing inhomogeneities. An approximation for applying the Monte Carlo data to the configuration where the lateral extent of the inhomogeneity is less than the beam area, is also presented. These new Monte Carlo techniques rely on integration over the dose distributions from constituent sub-beams of small area and the accuracy of the method is thus independent of beam size. The power law correction equation (Batho equation) describing the dose distribution in the presence of tissue inhomogeneities is derived in its most general form. By comparison with Monte Carlo reference data, the equation is validated for routine patient dosimetry. It is explained why the Monte Carlo data may be regarded as a fundamental reference point in performing these tests of the extension to the Batho equation. Other analytic correction techniques, e.g. the equivalent radiological path method, are shown to be less accurate. The application of the generalised power law equation in conjunction with CT scanner data is discussed. For ease of presentation, the details of the Monte Carlo techniques and the analytic formula have been separated into appendices.
一个蒙特卡罗计算机程序已被用于计算 60Co 外照射束与含有不均匀性的介质相互作用产生的轴向和离轴深度剂量分布。还提出了一种将蒙特卡罗数据应用于不均匀性横向范围小于射野面积的配置的近似方法。这些新的蒙特卡罗技术依赖于对小面积组成子射野的剂量分布进行积分，因此该方法的准确性与射野大小无关。描述存在组织不均匀性时剂量分布的幂律校正方程（巴托方程）以其最一般的形式推导得出。通过与蒙特卡罗参考数据比较，该方程在常规患者剂量测定中得到验证。解释了为什么蒙特卡罗数据可被视为在对巴托方程进行这些扩展测试时的一个基本参考点。其他解析校正技术，如等效放射路径法，被证明准确性较低。讨论了广义幂律方程与 CT 扫描仪数据结合的应用。为便于表述，蒙特卡罗技术和解析公式的细节已分别列入附录。
