Carbon fiber couch inserts reduce set-up errors caused by sagging in radiotherapy treatment couches. Studies have described such inserts as radio translucent with negligible radiation field attenuation. There appears to be no evidence in the literature that the thickest region affects attenuation and dose distribution during external beam radiotherapy. The majority of these tests were carried out with the radiation field normally incident on the center of the couch. This study evaluated and improved the calculated dose attenuation for the under-couch fields.
At normal incidence, the couch attenuated 6 MV photons by 10.5%,9.3%, 8.9%, and 8.6% for 5 × 5 cm2, 10 × 10 cm2, 15×15 cm2and 20×20 cm2 field sizes, respectively. We further found attenuation to be gantry angle dependent, with the highest attenuation recorded at 180° – at which angle the couch attenuated the 6 MV photon beam by 10.5% for the 5 × 5 cm2 and 9.3% for the 10 × 10 cm2 field sizes. Similarly, a 10 MV photon beam was attenuated by 8.6% and 7.7% for the 5 × 5 cm2 and 10 × 10 cm2, respectively, and 7.3% for both 15×15 cm2 and 20×20 cm2 field sizes, furthermore, a 15 MV was attenuated by 7.4%, 6.6%, 6.5% and 6.4% for 5 × 5 cm2, 10 × 10 cm2, 15×15cm2, and 20×20 cm2 field sizes, respectively. while the attenuation at 1600 and 1400 was less where the lowest attenuation recorded at 1400 at which angle the couch attenuated the 15 MV photon beam by 1.5.