Abstract:
To study the dosimetric parameters influenced in Exact couch and the Exact IGRT couch attenuation and to determine the effect in conventional, IMRT and VMAT treatment techniques. The dose measurements were employed in both energies of 6 MV and 10 MV photon beams with field sizes of 3×3 cm², 5×5 cm², 10×10 cm², and 20x20 cm² for various gantry angles around the treatment couch at cranial, middle and caudal couch position. The effect of couch attenuation in conventional, IMRT and VMAT treatment techniques were investigated. The maximum attenuation was detected at 6 MV with the smallest field for all couch positioning setups. The Exact couch showed maximum attenuation of 18.5% for 6 MV and 14.7% for 10 MV at 170 and 190 degree at rail in and cranial couch position, while the Exact IGRT couch presented the maximum attenuation of 5.1% for 6 MV and 5.0% for 10 MV at 140 and 220 degree at caudal position for 3x3 cm² field size. The maximum percent dose differences between measurement and calculation without couch consideration for 6 MV in parallel opposing fields of chest treatment were 7.96% for Exact couch at rail in position and 2.29% from single field PA spine treatment for IGRT couch. The percent dose differences in IMRT plans between measurement and calculation without couch were varied from 2.67% to 5.89% for Exact couch and 0.67% to 3.25% for Exact IGRT couch. While VMAT plans presented the deviation from 2.87% to 4.73% for Exact IGRT couch. The percent dose differences between measurement and calculation with couch were smaller than without couch for all techniques study. The effect from beam attenuation by the treatment couch is significant for patients treated with posterior or oblique posterior fields. The dose deviation due to couch attenuation of Varian couch depends on the field size, angle beam, energy, couch position and type. Therefore, patients should be avoided from the beam pass directly through rail position in posterior and posterior oblique or the correction should be included in treatment planning systems.
