SU‐E‐T‐792: Dosimetric Influence of Carbon Fiber Couchtop on Volumetric‐Modulated Arc Therapy

Purpose: To evaluate the dosimetric influence of carbon fiber couchtop on Volumetric‐modulated arc therapy (VMAT). Methods: In this study, a carbon fiber couchtop has been incorporated into Pinnacle3 9.0 treatment planning system, with the help of model‐based segmentation (MBS) function. This was done by generating the couch top's model from contours of the couchtop, together with the density information. Ten cases previously treated with VMAT in our institute were selected, six head‐and‐neck (HN), two thoracic, two abdomen cases. For each case, a VMAT plan was optimized without the couchtop model, and the final dose calculation was performed twice, with (WI plan) or without (WO plan) the couchtop model. Dual arcs with full or partial gantry rotation were used according to each case's geometry. The MU of WI plan was set to the same value of the WO plan for comparison. Dose indices for targets and OARs were determined from dose—volume histograms.Results: The comparison for the 10 cases showed that the WI plans achieved worse target volume coverage than WO plans, with an average difference of 3.3% in V100% (target volume that receives 100% of the prescription dose). Especially for small boost volumes, the difference larger that 11% in V100% was observed. The largest difference in mean dose of the target volumes was 1.0 Gy. For almost all the OARs, the mean doses for WI plan were smaller than those for WO plans Conclusions: In conclusion, the carbon fiber couchtop significantly affects the dose delivered to the target volume(s) in volumetric‐modulated arc therapy, and should be included in treatment planning to avoid any discrepancy between the prescribed dose and delivered dose.