Metal artifacts cause errors in the exact delineation of implants and dose changes in radiotherapy. In this study, the dose distribution differences in the region of interest (ROI) were calculated by deliberately making contouring errors from the real size of model implants by using both Monaco treatment planning system (TPS) and Geant4 toolkit. In Sec. 2, the computed tomography images were acquired by placing known uniform cylindrical geometry titanium (Ti6Al4V) and cobalt (CoCrMo) alloys into water phantoms separately. The metal alloys were artificially contoured as 2 mm contracted and expanded from their real dimensions in Monaco TPS. The plans were generated with 6 MV photon beams for contouring of three different sizes, real, contracted and expanded, for each metal alloy. In addition, all configurations were simulated in Geant4 by using the photon energy spectrum data of the Elekta Synergy linear accelerator. Then, the 3D dose data obtained from ROIs near the implant in Monaco TPS and Geant4 were analyzed with in-house programs. In Sec. 3, the depth dose values of Geant4 were compatible with TPS calculations and ion chamber measurements. When the alloys were contoured to real dimensions, it was observed that the local isodose values have changed up to 15% in ROI. The mean dose values were found to be higher in contracted and lower in expanded contours. It was observed that ±2 mm error in contouring the implants changed the mean dose up to ±8%. In Sec. 4, this study emphasized that a few millimeters of error in contouring different implant materials can have a significant effect on dose distribution in a region close to the implant.