We report an investigation on the accuracy of inhomogeneity correction in photon radiotherapy from CT scans with different settings. Specifically, the dosimetric differences from different CT scan parameters (kV, mAs) to phantoms and from different Hounsfield unit versus electron density (HU-ED) curves to patients are investigated. The absolute dose per monitor units (dose/MU) is used to quantify the results. We found that only for high-density bones (cranium, femoral tube, etc) using small field 18 MV beams, the dose/MU is up to 2% higher for CT scans using 80 kV than for 130 kV at a depth just beyond the bone and is up to 1-1.5% higher for CT scans using 80 mAs than for 300 mAs. For low-density bones (such as femoral head) and lung, the difference is 1% or less with different kV or mAs settings. The dose/MU varies with different HU-ED curves by up to 2%. The HU-ED curve from the stochiometric calibration was found to be more accurate based on a real measurement. A simplified 4-point curve provides nearly the same accuracy as the stochiometric calibration and may be used as an alternative for routine clinical application.