In recent years, carbon fibre couch tops have become popular in radiation therapy with marketed attributes such as high tensile strength, light weight, and relative radiotranslucence. We have performed a detailed analysis of the iBEAM evo couch top using two models created in the Pinnacle v9.0 treatment planning system. The first model is a single ROI, and its thickness and shape was determined based on attenuation factor measurements in cylindrical phantoms. The second model is a thin shell made of two ROIs with exact outer dimensions as the real couch top. The couch top attenuation was found to range from 2% to 6.5% for a 6 MV beam energy. The two models were found to match the attenuation factor measurements to within 4.2% and 1.7%, respectively. The clinical impact of the couch top models was examined in 4 VMAT and 4 IMRT prostate plans by comparing mean and maximum point doses, DVHs, and measurements (VMAT plans only) using Gamma analysis. A very small, but clinically insignificant difference was observed between the two couch top models for both VMAT and IMRT plans. The lack of a couch top model in treatment planning was also examined and found to result in ∼ 1% average underdose to the PTV. This study suggests that the iBEAM evo couch top is relatively radiotranslucent for typical VMAT and IMRT delivery as long as the number of control points with extremely oblique beam angles is relatively small.