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Background: It is essential to evaluate the accuracy of dose calculation for treatment planning systems (TPSs). This study's primary goal was to evaluate the accuracy of dose calculation for ISOgray TPS in the presence of a wedges in the treatment fields. Materials and Methods: GATE (Geant4 Application for Tomography Emission) as a Monte Carlo (MC) code was utilized to model the 6 MV photon beam of an Elekta Compact linac. It did MC code verification for three different field sizes and three depths for open, and wedged fields with gamma index tool. Following the confirmation, the percentage depth dose (PDD) and dose profile were calculated using the TPS and compared with the simulation results. In the next step, the TPS dose calculations for the 10×10cm2 field with different wedge angles were compared by the result from analytical formula. Results: The PDD and dose profiles for open fields met the gamma index criteria. However, there was disagreement for large wedged fields. The dose profiles of wedge angles using Petti analytical equation were compared to ISOgray dose profiles. Results showed that dose profile points with all wedge angles meet the gamma index criteria except for the 45° wedge angle. Conclusions: The results indicated that the disagreement between MC and TPS dose calculations increases by increasing wedge angle and field size. The uncertainty is due to TPS dose calculation algorithm causing noticeable disagreement. A MC-based TPS for dose calculation is recommended to reduce the error in dose calculation or at least medical physicist consider this issue. [ABSTRACT FROM AUTHOR]