The variation of Hounsfield unit and relative electron density determination as a function of KVP and its effect on dose calculation accuracy
Ohl, Aaron F.
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The purpose of this study was to investigate the effect that differences in relative electron density for different kilovoltage peak (kVp) settings have on radiotherapy dose calculations. A Nuclear Associates 76-430 Mini CT QC Phantom with materials of known relative electron densities was imaged by one 16-slice CT scanner. The Hounsfield unit (HU) was recorded for each material with energies ranging from 80 to 140 kVp and representative relative electron density (RED) curves were generated inside the treatment planning system (TPS). A virtual cubic water phantom with a 5 cm thick inhomogeneity located 5 cm deep was constructed to measure dose differences between calibration curves. The inhomogeneity was assigned specific HU to be evaluated for each calibration curve. The dose was then calculated at points within and below the inhomogeneity and compared to doses obtained with the 80 kVp curve. The corresponding dose calculations revealed the largest dose differences inside the inhomogeneity of 1.6% for aluminum and 2.2% for bone. Beyond the inhomogeneity dose differences of 6.8% for aluminum and 7.6% for bone were measured. The large dose differences in the phantom study prompted a study involving three clinic patients with three anatomies of interest: lung, prostate, and metal hip prosthesis. The dose difference in the anatomies of interest was largest in the prosthesis patient with a maximum of approximately 3.5%. For materials with large relative electron density values (excluding Teflon, due to its chemical composition), the dose differences in the clinical and phantom calculations were significant. This result may indicate that implementing energy specific RED curves can increase dose calculation accuracy in those materials with large relative electron densities.