Computed tomography (CT) images contain information of relative electron density (RED) of tissues in terms of CT numbers and hence are utilized for tissue inhomogeneity corrections in radiotherapy dose calculations. The present study has determined the effect of tube voltage and current on CT number to RED calibration (CT sensitometry) curves. Catphan phantom containing materials of REDs from 0.001 to 1.868 has been scanned at tube voltage from 80 kV to 140 kV and tube current from 100 to 475 mA. CT numbers of all the materials except Teflon has been found to increase with tube voltage. Variations up to 80 HU has been observed with voltage. Variations with tube current were random. CT numbers changed by higher values at low voltage than at higher voltages. The radiation dose varied by less than 1 % for different curves used for dose calculation in 3DCRT, VMAT and SRT treatment plans of different anatomical sites. Dose differences decreased with increasing plan complexity. Changes in dose distributions near air cavities and bones have been observed. Variations of CT sensitometry has been found to be statistically significant, however, its dosimetric impact was statistically and clinically insignificant.

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