SU-E-T-489: An Investigation of the Effect of Couch Bar Position on Dose Delivered with IMRT and Rapid-Arc

Abstract

Purpose: To investigate the effect of different couch bar positions on dose delivered with IMRT and Rapid‐Arc. Methods: An ArcCheck with two 0.3cc ion chambers was used for dose measurements with static beam IMRT and Rapid‐Arc. One chamber was placed at the ISO, in the center of the ArcCheck at d = 13.3 cm, and another at a point on ArcCheck surface which was at the 5 o'clock position. IMRT plans in this work had five to seven beams, and at least two of these beams were at gantry angle between 0 and 60, or 300 and 360 degree. Rapid‐Arc treatments had two arc‐beams from 1.0 to 359 degree. The treatment couch was the Varian Exact Couch. The bar was placed at three positions (open, closed or at the middle) to study the effect on doses delivered for the same plan. Eleven clinical cases with 6 or 10 MV photon beams were investigated. Results: For static beam IMRT, doses delivered at ISO for different bar positions for the same plan was only moderately changed by 0.2 to 3.2% (average 1.5%) as a function of bar position. Surface dose increased between 6.8% and 15.6% when the beam passed through the bar that served as a beam build‐up. For Rapid‐Arc treatments the gantry is continuously moving, and it does not deliver large MUs at any fixed beam angle, therefore the effect of bar position is reduced. Dose at ISO was attenuated by 0.2 to 1.2 % (average 0.5%), and surface dose increased from 1.5% to 4.9%. Conclusions: Our measurements show that different bar positions affect dose delivered in treatments, particularly surface dose at some areas with static beam IMRT. This effect is considerably smaller for Rapid‐Arc.