Background and Objectives: The difference of calculated dose by treatment planning system (TPS) and real dose distribution in patient body is related to calculating algorithms of TPS, and this difference is more evident in heterogeneous tissues like lung. In this study dose distribution was calculated by two algorithms of Isogray TPS and compared to dose distribution calculated by Monte Carlo simulation in water and lung.Subjects and Methods: In this study, the percentage depth dose (PDD) curves resulting from different algorithms of Isogray TPS were compared by data from simulation in homogeneous water tissue. The difference between dose distribution calculated by Isogray TPS algorithm and CT2MCNP program were obtained and accuracy of mentioned algorithm were evaluated in lung.Results: Thedifference between simulation results and results of TPS algorithms including collapsed cone, Fast Fourier Transform (FFT) convolution and superposition in heterogeneous lung tissue was calculated  5%, 6% and 7%, respectively. In the maximum dose, The obtained difference  was 7 % with  the same algorithms. no significant differences in the water between simulation results and results of TPS algorithms were observed. and the difference was obtained less than 1%.Conclusion: Inclinical treatment, there was significant difference in dose calculation by TPS algorithm in heterogeneous lung tissue. But this difference was negligible in homogeneous tissues