Background:A conversion from computed tomography(CT)values obtained using single-energy CT to relative electron density(RED)for water or mass density(MD)is important process for calculating a dose distribution in human body.However,the dependence on body size for CT value could affect of the conversion of RED(MD)as well as the dose calculation.Purpose:The purpose of this study is to evaluate of impact of CT value variance caused by phantom size for calculating dose distribution in human body.Materials and methods:We employed GE Discovery CT750 HD CT scanner.Scan parameters were 120 kVp with non-helical scan mode.Four kinds of CT-RED(MD)tables were acquired by scanning tissue substitutions(Gammex-RMI 465)inserted in the four forms of water equivalent phantom(10 cm and 20 cm diameter,22 cm x 30 cm and 30 cm x 40 cm ellipses).CT-RED(MD)dates were forwarded to radiation treatment planning system:Eclipse Ver.13.6.The dose distribution for head,neck,prostate and lung cancer cases were calculated by Analytical Anisotropic Algorithm(AAA)and Acuros XB(AXB)in external photon beam radiotherapy.Results:The differences of RED which were calculated by four patterns show as follow,lung substitute(0.280):0.06,soft tissue substitutes(0.923-1.072):0.01-0.04,bone substitute(1.46):0.12.The differences of the dose distribution for three cancer cases calculated by two algorithms using four CT to RED tables were less than 0.1%.Conclusions:RED and MD were affected by impact of CT value dependent on phantom size.There was not found the effect for calculating the dose distribution.