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.
