DETERMINATION AND DOSIMETRIC EVALUATION OF HU CALIBRATION CURVES IN EXTENDED ACQUISITION FOR THE ACUROS XB ALGORITHM

The dose calculation algorithms used in radiotherapy use a calibration curve to convert HU (Hounsfield Unit) to mass and electronic density, making it possible to differentiate tissues in interaction with the beam. The maximum HU value for normal CT image acquisitions is 3071, which is the value reached for all high density materials due to saturation, not allowing the differentiation of, for example, metallic materials. Currently, CT scanners allow extended mode CT acquisition, which features a larger HU scale, differentiating high density materials in the calibration curves. In the Eclipse planning system, Acuros XB is available, which is very accurate and close to Monte Carlo simulations, having advantages in its application in clinical radiotherapy. The objective of this work was to determine the HU calibration curve for CT Optima (GE) CT by varying image parameters such as normal and extended acquisition, beam energy, electrical current generated in tube, Field-of-View (FOV) and axial resolution, evaluating the dosimetric impact caused. CT images of two simulator objects containing different materials were acquired: titanium, cobalt chromium, stainless steel, aluminum, acrylic, polystyrene; and the HU number of each structure was determined for each CT acquisition, varying the parameters mentioned. From this, the HU calibration curve was determined by plotting HU number by mass density. Dosimetric comparison was made using the Acuros XB algorithm in a phantom with titanium, cobalt chromium and aluminum metal insertion in normal and extended CT acquisition modes, analyzing the dose distribution with the two calibration curves. There was a significant variation in the number of HU for different energies, FOVs and the normal to extended acquisition mode. Extended acquisition has HU values equal to normal acquisition for materials with density <3g / cm³. For higher densities, there is a differentiation of HU, where the mean value for titanium was 9018 ± 784 and for cobalt chromium 27500 ± 2500, being up to ten times higher than the maximum HU value of normal acquisition. Acuros XB therefore predicts a 4.4% higher dose absorption in aluminum volume, 15% higher in titanium and 10% higher in cobalt chromium when using the extended CT calibration curve compared with the normal CT calibration.