Varian Halcyon (HA) linear accelerators (LINAC) require kilovoltage cone-beam computed tomography (CT) acquisition for all patients before all treatment fractions. The aim of this study is to compare dose indices of various available protocols with different calculation and measurement methods. CT dose index (CTDI) in mGy is a measure of the radiation dose output of a CT scanner. A pencil ionisation chamber was used to measure dose index free air and in a standard CTDI phantom for different imaging protocols of HA and TrueBeam LINACs. For the point measurements, we found large deviations between displayed and calculated low CTDI values: 26.6 and 27.1% at Head low dose and Breast protocol, respectively. The calculated values were always larger than the displayed ones for all protocols and measurement setups. For the point measurements similar results were found as in the international literature, where the displayed values the measured CTDIs.
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