Dosimetric Impact of the CT-ED Curve in Radiotherapy of Pediatric PatientsMikić, Milica (Avtor) Simončič, Urban (Mentor) Peterlin, Primož (Komentor) radiotherapy of pediatric patientscomputed tomographyCT-ED curveHounsfield unitsdosimetrytreatment planning systemdose-volume histogramgamma index analysisThe electron density on the tomogram is normalized in CT-numbers. An experimentally obtained CT-ED curve is used to determine the electron density (ED). The relationship between the CT number and the electron density of the material/tissue depends on the voltage of the cathode tube. In radiotherapy, we usually use the relationship between the CT-number and the electron density of the substance obtained at 120 kV, which corresponds to most cases. Pediatric patients are an important exception. They are often imaged at lower tube voltages (70 kV or 80 kV), which contributes to better soft tissue contrast but also results in a somewhat lower received dose. Nevertheless, a standard conversion table between CT-number and electron density is used in treatment planning, resulting in an incorrect (too high) electron density being assigned to the CT-number for materials or tissues with higher electron density. The dosimetric impact of this error was calculated in this study. Pediatric patients under 14 years of age treated at the Department of External Radiation Therapy of the Oncology Institute in Ljubljana were included in this study. The group size was 32 patients. The cases of pediatric patients imaged with Siemens Somatom Definition AS and Philips Brilliance Big Bore scanners were collected, and their CT images were exported and anonymized. The CT-ED tables for different tube voltages were obtained from measurements on phantoms. A computer program was written to read images in DICOM format and correct them with respect to different X-ray tube voltages (80, 90, 100, and 120 kV). The modified CT images were imported into the radiation planning system, the existing clinical radiation plan was mapped onto them, and the spatial distribution of absorbed dose was calculated. The absorbed dose spatial distributions for each patient (actually used and calculated to 120 kV) were exported and anonymized. Finally, differences in absorbed dose calculated on the original and modified CT images were analysed. For low densities up to 1.1 g/cm3, Hounsfield unit (HU) differences of 0–3% were found, but for densities in the range above 1.1 g/cm3, HU variations as a function of X-ray tube voltage of up to 31% were found. The maximum calculated difference in absorbed dose in the tumour (i.e. planning target volume) proved to be less than 1%, while in critical organs it was less than 2.2%. Therefore, the error due to the use of an incorrect CT-ED conversion curve does not critically affect the dose distribution.20232023-01-15 08:15:02Magistrsko delo/naloga143859sl