Improving ion beam therapy treatment planning formetal implants by using dual-energy CT scanning

Abstract

One of the major uncertainties in ion beam therapy planning is the calculation of ion ranges in the patient’s tissue from CT images. The presence of non-tissue-equivalent mate- rials and materials with high photon attenuation like metals may aggravate this problem. Dual Energy Computed Tomography (DECT) allows to compute the electron density and effective atomic number. It could already be shown that this additional material information enables a more precise calculation of ion ranges. This thesis investigates the feasibility of the DECT approach for a range of metals from aluminum (Z = 13) up to tungsten (Z = 74). DECT scans of the samples reconstructed with a 16 bit CT scale and raw data based beam hardening correction were analyzed. The electron density and effective atomic number of aluminum and titanium (Z = 22) could be determined within the range of a few percent. These quantities could not be determined for samples with Z ≥ 22, but the samples were distinguishable from each other by their different CT numbers up to molybdenum (Z = 42). The precision of the determined ion ranges could be improved for aluminum from -11.46% to 4.88% and for titanium from -36.4% to 2.75% compared to ion range estimations from 120 kV CT. The size of nearly all metal samples could be assessed from the images with precision in the range of the voxel size of 0.6 mm. Streaking artifacts around the samples were minor for aluminum and titanium. For materials with Z ≥ 26, severe artifacts could be observed. The samples were investigated with Mega Voltage Computed Tomography to compare DECT with this rivaling method. It was found that MVCT yielded superior results in case of materials with Z ≥ 26. However, DECT offers in clinical routine the advantage of faster scanning times and greater technical maturity of the scanner. Discriminant analysis was tested as an alternative way to obtain ion ranges from Dual Energy CT images without physical model. Only small mean absolute deviations from reference ion ranges were observed for an animal sample.