Poster - Thurs Eve-34: Extended CT-range in RT planning of pelvic cancer treatment in presence of hip replacements

Extended CT range in conventional CT scanners has a potential to allow for a more conformal treatment of patients with hip prosthesis. Its use may limit inaccuracies in electron density maps that are observed due to severe artifacts in CT data. In this study, we investigate the use of CT images with extended CT numbers in dose calculations and compare the results of calculations with standard CT data and measured doses. A phantom containing a hip prosthesis was scanned and treatment was planned with extended and standard CT numbers. The density override function was used to eliminate the effect of artifacts in the region outside of the metallic implant, while raw CT numbers were used to indicate density within the implant. Dose measurements were performed with two types of ion chambers at 6, 10 and 18MV energies. Our results indicate that data with extended CT range result in a better agreement between measured and calculated dose at the central position of the body phantom, as should be expected. However, CT artifacts within the implant region also result in discrepancies between the measured and calculated dose. The discrepancy is greater at lower cross-sectional thickness where bright, high density surface artifacts are high relative to the artificially low density inner region of the implant. Potential ways of resolving the discrepancies are outlined and a possibility of their application to clinical routine will be discussed.