Influence of iodine contrast agent on the range of ion beams for radiotherapy.

The basis for the range calculation of heavy ions in tissue is an empirical correlation between x-ray CT numbers and ion ranges measured for tissue equivalent materials. Iodine contrast agents (CA), used during computed tomography (CT) imaging, lead to an increase of the Hounsfield units in tissue with increased CA uptake and cause errors in the calculation of the ranges. The aim of this work is to quantify how accurately ion range is calculated in CA loaded tissue. In order to quantify the mean change in Hounsfield units (HU), a statistical analysis of 25 CT data sets with and without CA was performed. To establish a relation between the change in Hounsfield units due to CA and changes in ion range, the Hounsfield number for various CA concentrations and the range of ions in CA was measured. The analysis of CT data sets showed that after intravenous injection of 100 ml contrast agent (Imeron300) at a concentration of 300 mg iodine/ml an increase of the Hounsfield numbers in tumor tissue of up to 57 HU can be observed. The measured range shift in CA is much smaller than calculated by the treatment planning system (TPS). The maximum error in range resulting from the CA enhanced data is approximately 2.5% and results mainly from this wrong interpretation of HU by the TPS. For a tumor with an extent of 5 cm this leads to an exaggeration of the ion ranges during irradiation of (1.24+/-0.04) mm. This may be clinically relevant in cases where highest precision is needed and where organs at risk are close to the target volume. In view of these findings it may be safer to rely solely on native CT data for the purpose of dose and range optimization in therapy planning for heavy ions and protons.