A 3D superposition pencil beam dose calculation algorithm for a ⁶⁰Co therapy unit and its verification by MC simulation

The MCNP Monte Carlo code was used to simulate the collimating system of the ⁶⁰Co therapy unit to calculate the primary and scattered photon fluences as well as the electron contamination incident to the isocentric plane as the functions of the irradiation field size. Furthermore, a Monte Carlo simulation for the polyenergetic Pencil Beam Kernels (PBKs) generation was performed using the calculated photon and electron spectra. The PBK was analytically fitted to speed up the dose calculation using the convolution technique in the homogeneous media. The quality of the PBK fit was verified by comparing the calculated and simulated ⁶⁰Co broad beam profiles and depth dose curves in a homogeneous water medium. The inhomogeneity correction coefficients were derived from the PBK simulation of an inhomogeneous slab phantom consisting of various materials. The inhomogeneity calculation model is based on the changes in the PBK radial displacement and on the change of the forward and backward electron scattering. The inhomogeneity correction is derived from the electron density values gained from a complete 3D CT array and considers different electron densities through which the pencil beam is propagated as well as the electron density values located between the interaction point and the point of dose deposition. Important aspects and details of the algorithm implementation are also described in this study. - Highlights: • Monte Carlo simulation of photon and electron spectra for a ⁶⁰Co therapy unit. • Monte Carlo simulation of a polyenergetic Pencil Beam Kernel (PBK). • Inhomogeneity correction based on Monte Carlo simulation. • Implementation of dose calculation algorithm on the CT matrix with 3D inhomogeneity correction