Assessment of CT Imaging Protocols Impacts on Calculation of Point Dose in Water Phantom using Radiotherapy Treatment Planning System

Point dose calculation in the Treatment Planning System (TPS) is performed using Computed Tomography (CT) images because CT images data have the tissue electron density information. The effect of CT imaging protocols on the calculation of point doses in TPS is one of the most important subjects that was evaluated in this study.

CT scan imaging was performed from cylindrical water phantom using three scanner systems and different imaging technical parameters. The CT images data were irradiated in TPS to delivering a 200 cGy radiation dose to the center of the phantom with 6 and 15MV X-Ray photon energy with multiple radiation fields and Monitor Unit (MU) were separately calculated. In the TPS, a virtual water phantom with the same characteristic as CT image phantom was simulated and irradiated with similar conditions. The difference in MU values obtained from two irradiation methods in TPS was compared with Wilcoxon nonparametric test.  

Variations of mA, kV, Pitch, slice thickness, and kernel as CT imaging parameters have not significantly affected radiotherapy point dose calculation (<2%). CT imaging protocols as a thin slice, 80 kV, and sharp kernel have the greatest difference between CT image-based calculation and designed phantom calculation in TPS where wedge field and 6 MV photon energy were used.

The use of CT images obtained with multiple protocols can be used without having a significant effect on the dose calculations of the treatment planning system.