Influence of CT contrast agents on dose calculations in a 3D treatment planning system

In treatment planning for conformal radiotherapy, it is possible to attain high accuracy in contouring the outline of the target volume and organs at risk by giving contrast agents (CAs) during the CT scan. In order to calcul ate the dose from the CT scans, Hounsfield units (HUs) are converted into t he parameters of a standard set of tissues with given atomic composition an d density. Due to the high atomic number of contrast media, high HU values are obtained during CT scanning. The Helax treatment planning system, for i nstance, erroneously takes them for high density tissue. This misinterpreta tion results in high absorption of high-energy photon beat ns and thus affe cts the dose calculation significantly. A typical bolus diameter of 3 cm an d HU values of 1400 cause an overdose of up to 7.4% and 5.4% for 6 MV and 2 5 MV photon beams, respectively. However, since the CA concentration and it s expansion are rather low the effect on dose calculation in treatment plan ning is negligible.