The relationship between the dose calculated with a radiotherapy treatment planning system (RTPS) and CT number verses the relative electron density curve was investigated for various CT voltages and beam qualities. We obtained the relationship between the CT numbers and electron densities of the tissue equivalent materials for various CT voltages and beam qualities. At lower CT voltages, the higher density materials, like cortical bone, showed larger CT numbers and the soft tissues showed no variations. We performed a phantom study in a RTPS, where a phantom consisted of lung and bone regions in water. We calculated the dose received behind the lung and bone regions for 6 MV photon beams, in which the regions below the lung, water and bone received higher doses in this listed order. The result was the same for 10 MV photon beams. For the clinical application, the doses were calculated for the lung and pelvis. No difference was observed when using different electron density conversion tables with various CT voltages from a same CT. A relative dose difference of 1.5% was obtained when the CT machine for the density conversion table was different from that for the CT image for planning.

KeywordsRadiotherapy treatment planning system (RTPS), Computed tomography (CT), Electron density phantom, CT voltage