Calibration of CT densityvalues in dosimetry verification of intensity modulated radiation therapy

：Objecfive Based onintensity modulated radiation therapy (IMRT)phantom,the impact of CT-to-density conversioncurve on dosimetry verification of IMRT is investigated and calibrated.Methods Theelectron density phantom was used to establish the CT-to-density conversion curve inradiation treatment planning system.IMRT plans of 12 nasopharynx carcinoma patients werechosen,copied to IMRT phantom and computed for the dose distribution.For each plan ameasured point was put at the place where the dose was well-distributed and its dose valuewas measured using the ionization chamber.The physical density of IMRT phantom and itsCTvalue were input into the planning system,to make a calibration for the CT-to-densityconversion curve.The dose distribution was recomputed for each IMRT plan.Other parameterswere kept the same in the plans and the differences between the computed dose valuesbefore and after correction were compared with the measured values.Results In 12nasopharynx carcinoma IMRT plans,the average error of computed dose values was 1.96%±0.87%before correction and 0.63%±0.74%after correction,compared with measured values.The errorbetween measured values and computed values after correction was less than ±2% whereasthe maximum error of computed values before correction was 3.24%.Conclusions The computeddose values are closer to the measured values when using the calibrated CT-to-densityconversion curve.The CT density values of IMRT phantom should be verified before usage,soas to increase the accuracy of IMRT dosimetry verification.