Ahnesjö A
Department of Radiation Physics, Karolinska Institute, Stockholm, Sweden.
Med Phys. 1989 Jul-Aug;16(4):577-92. doi: 10.1118/1.596360.
A method for photon beam dose calculations is described. The primary photon beam is raytraced through the patient, and the distribution of total radiant energy released into the patient is calculated. Polyenergetic energy deposition kernels are calculated from the spectrum of the beam, using a database of monoenergetic kernels. It is shown that the polyenergetic kernels can be analytically described with high precision by (A exp( -ar) + B exp( -br)/r2, where A, a, B, and b depend on the angle with respect to the impinging photons and the accelerating potential, and r is the radial distance. Numerical values of A, a, B, and b are derived and used to convolve energy deposition kernels with the total energy released per unit mass (TERMA) to yield dose distributions. The convolution is facilitated by the introduction of the collapsed cone approximation. In this approximation, all energy released into coaxial cones of equal solid angle, from volume elements on the cone axis, is rectilinearly transported, attenuated, and deposited in elements on the axis. Scaling of the kernels is implicitly done during the convolution procedure to fully account for inhomogeneities present in the irradiated volume. The number of computational operations needed to compute the dose with the method is proportional to the number of calculation points. The method is tested for five accelerating potentials; 4, 6, 10, 15, and 24 MV, and applied to two geometries; one is a stack of slabs of tissue media, and the other is a mediastinum-like phantom of cork and water. In these geometries, the EGS4 Monte Carlo system has been used to generate reference dose distributions with which the dose computed with the collapsed cone convolution method is compared. Generally, the agreement between the methods is excellent. Deviations are observed in situations of lateral charged particle disequilibrium in low density media, however, but the result is superior compared to that of the generalized Batho method.
描述了一种光子束剂量计算方法。初级光子束通过患者进行光线追踪，并计算释放到患者体内的总辐射能量分布。利用单能核数据库，根据束流光谱计算多能能量沉积核。结果表明，多能核可以用(A exp( -ar) + B exp( -br)/r2)进行高精度的解析描述，其中A、a、B和b取决于相对于入射光子的角度和加速电位，r是径向距离。推导了A、a、B和b的数值，并用于将能量沉积核与每单位质量释放的总能量(TERMA)进行卷积以产生剂量分布。通过引入坍缩锥近似来促进卷积。在这种近似中，从锥轴上的体积元释放到等立体角的同轴锥中的所有能量，都沿直线传输、衰减并沉积在轴上的元中。在卷积过程中隐含地对核进行缩放，以充分考虑受照体积中存在的不均匀性。用该方法计算剂量所需的计算操作数与计算点数成正比。该方法针对5种加速电位(4、6、10、15和24 MV)进行了测试，并应用于两种几何形状；一种是组织介质平板堆栈，另一种是类似纵隔的软木和水的体模。在这些几何形状中，EGS4蒙特卡罗系统已用于生成参考剂量分布，并与用坍缩锥卷积法计算的剂量进行比较。一般来说，两种方法之间的一致性非常好。然而，在低密度介质中横向带电粒子不平衡的情况下观察到偏差，但结果比广义巴托方法的结果要好。
