Inter-comparison of phantoms for CT numbers to relative electron density (RED)/physical density calibration and influence to dose calculation in TPS

Six different phantoms (2x CIRS, 1x Agmeco, 1x QUASAR, 1x Gammex, 1x CatPhan) and Varian formulas were used to evaluate their suitability for CT numbers to relative electron density (RED) calibration in the treatment planning system at two centres using two CT scanners. The variability in calibration curve and the influence to dose calculation in the TPS were estimated for 6 MV and 18 MV. The presence of scattering material around inhomogeneities and influence of CT spectra (90 kV, 120 kV, 140 kV) were evaluated. All phantoms were found convenient for calibration except CatPhan where large deviations were observed for dense bone. Phantoms without scatter material around inhomogeneities should be used with caution. Varian formula slightly overestimates calibration curve in the region of 200 - 300 HU. The tolerance level of 10% suggested for RED during commissioning is reasonable as it can cause change in dose of 3% and 1.5% for 6 MV and 18 MV respectively as a very conservative estimate. Calibration curves should be acquired for all scanning protocols with different CT spectra.