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 calculate the dose from the CT scans, Hounsfield units (HUs) are converted into the parameters of a standard set of tissues with given atomic composition and density. Due to the high atomic number of contrast media, high HU values are obtained during CT scanning. The Helax treatment planning system, for instance, erroneously takes them for high density tissue. This misinterpretation results in high absorption of high-energy photon beams and thus affects the dose calculation significantly. A typical bolus diameter of 3 cm and HU values of 1,400 cause an overdose of up to 7.4% and 5.4% for 6 MV and 25 MV photon beams, respectively. However, since the CA concentration and its expansion are rather low the effect on dose calculation in treatment planning is negligible.