In radiotherapy treatment couches, rigid carbon fiber couch inserts reduce set-up errors caused by couch sagging. These inserts have been described in several studies as radio-translucent with negligible radiation field attenuation. The majority of these tests were carried out with the radiation field normally incident on the center of the couch, and there appears to be no evidence in the literature of the effect of the thickest region (edge of the couch) on the attenuation and dose distribution during external beam radiotherapy. In this study, we evaluated and improved the calculated dose attenuation for the under-couch fields to reduce the skin surface dose. Using the difference between the measured and calculated attenuation doses obtained using the ion chamber and the “TPS” treatment planning system; an algorithm that identifies the influence of couch attenuation on patient dose verification. According to our results, the attenuation is affected by the gantry angle, where the maximum attenuation was at 180°, the angle at which the couch attenuates the 6 MV photon beam by 9.3 % for 10×10 cm2 field sizes, while the attenuation at 1600 and 1400 was less where the lowest attenuation recorded at 1400 at which angle the couch attenuated the 6 MV photon beam by 2.4. It would appear, therefore, that Connexion central opening couch with posterior beams results in significant decreases in the dose delivered to the target. The measured data were compared to the calculated values from the Monaco Treatment Planning System, and computed using the Monte Carlo (MC) and Collapsed Cone (CC) algorithms. The most minor agreement was observed at an angle of 1800 between the calculated and the measured attenuation for the 5 x 5 cm² field size with 6 MV and 10 MV photon beam energy where the difference was 5.3 % and 4 % for the MC and CC algorithm, respectively. Excellent agreement was observed at an angle of 160 between calculated and measured attenuation with the field sizes of 10 x 10 cm² for 6 MV, in which the difference (lose dose) was ±1.5 %.