| Dose calculation plays a vital role in a radiotherapy treatment planning system(RTS), whose precision has great influence upon the quality of the therapy plan druw up by radiation physicists. The research on fast and accurate dose calculation method is always one of the main contents of the RTS research work. Accurate, fast, quantifiable small section of the beam irradiation dose distribution is the basic requirements for IMRT dose calculation model in IMRT Treatment Plan System.Monte Carlo method is a stochastic arithmetic, which can accurately simulate particle transport in radiation therapy process and obtain the rigor dose result. Whereas its extensive application in clinic radiotherapy plan is limited for consuming too much time. With the continuous growth of demand on Intensity Modulated Radiation Therapy and the rapid development of computer speed, TPS system needs more rapid and accurate dose algorithm. Therefore improving and developing a fast and accurate method of dose calculation for Accurate Radiation Therapy System is of great significance.A finite-size Pencil Beam algorithm of matrix model based on Monte Carlo procedure was presented. The dose distribution of a series of finite size pencil beams with different incident angles simulated by Monte Carlo code were directly stored as matrix. The full beam dose distribution would be obtained by direct superposing. Some test examples including both regular and irregular field were verified by this algorithm with Monte Carlo simulation. Results showed that it can satisfy the requirement in accuracy and speed for IMRT TPS.The Batho inhomogeneity correction can modify the dose distribution of pure water to human body by the patient medical images like CT. Based on the resrarch of matrix model fsPB algorithm, the Batho inhomogeneity correction method is programmed, tested by the water-lung-water examples, compared with the result of the EGS simulation, which provides more choice for the dose calculation algorithm for IMRT TPS.
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