SU-E-T-549: On Importance of Accurate Treatment Couch Modeling in SmartArc VMAT Plans.

PURPOSE We investigate the impact of a treatment couch and its modeling in a treatment planning system on accurate dose delivery in Volumetric Modulated Arc Therapy (VMAT). METHODS A cone-beam CT of a Varian couch on a Varian Clinac 21EX unit, consisting of a carbon-fiber imaging couch top and supporting rails, was acquired, and a couch model was created in Philips Pinnacle 9.0 TPS based on the geometry of the couch top and the rails. The densities of the modeled couch top and the rails were based on the Hounsfield Unit and CT number-to-density information obtained from the CBCT scan. The densities were then adjusted based on the ion chamber measurements of a beam transmitting through the couch posteriorly at different gantry angles varying 1-5 degrees. SmartArc VMAT plans for patient treatment with and without the modeled couch were then generated and dose distributions were verified using the IBA MatriXX 2D ion chamber array. RESULTS We found that depending on the gantry angle the beam can be attenuated by up to 17% when going through the supporting rails, and up to 5% when passing through the couch top. Because of this, dose distributions of SmartArc plans were not accurate and did not pass the dose verification criteria without accurate modeling of the treatment couch. Adding the couch model remedies this problem and avoids underdosing the target. CONCLUSIONS We showed that the impact of a treatment couch in radiation therapy can be significant, and it is essential to include an accurate model of the couch in the treatment planning system, particularly for SmartArc VMAT plans in Philips Pinnacle TPS.