Changes in electron density relative to water due to the concentration of contrast agent in a ct simulation and dosimetric implications

Introduction Contrast agents for radiotherapy are used to improve accuracy in delineating organ at risk and treatment volumes. These agents modify Hounsfield Units (HU) and tissues electron densities relative to water (EDRW) what are used for determining absorbed dose in external radiotherapy by treatment planning systems (TPS). Purpose To evaluate changes in EDRW as a function of the concentration of the contrast agent (CCA) in an aqueous solution and to assess dosimetric implications. Materials and methods Different amounts of iodine contrast Omnipaque® 350 mg/ml of GE Healthcare are gradually introduced in a water phantom. The amount of contrast added is previously measured so each CCA is known. CT images were acquired using a clinical TC GE HiSpeed Nx/I for each concentration value. HU are measured by a 20 × 20 square ROI and transformed into EDRW by the characterized HU-to-EDRW calibration curves. EDRW value is plotted against the CCA. Percentage Depth Dose (PDD) is predicted by Eclipse TPS using the Analitytical Anisotropic Algorithm (AAA) v.11.0.31 in each aqueous solution and normalized to water in order to evaluate absorbed dose. Results EDRW increases linearly with the CCA. The normalized PDD shows an overdose of 1% when EDRW increases 1.5%, or 2% when EDRW increases 2,5% at 10 cm depth. Conclusion Using a CT with contrast in radiotherapy treatment planning as potentially source of error in the calculation of absorbed dose should be evaluated in each case. Disclosure No conflicts of interest to declare.