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

Abstract

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.

Effects of iodinated contrast media on radiation therapy dosimetry for pathologies within the thorax

“…Data from the patient in our study where there was left brachiocephalic vein obstruction indicated that larger, concentrated volumes of contrast media could influence the resultant dose distribution by more than ± 1.0%. While our results are comparable with those of Ramm et al 3 they are not entirely comparable with those of Weber et al, 4 where no evidence was found to suggest that the presence of larger, concentrated volumes of contrast adversely affected dosimetry. 4 As a consequence of this uncertainty, we recommend that patients whose thoracic neoplasms concentrate abnormal amounts of IVICM by disrupting the usual flow of blood through the vessels, should undergo both a non-contrast and contrast scan.…”

“…1,2 Treatment planning systems convert the HUs into corresponding electron densities in order to calculate dose and therefore HU values that yield incorrect electron densities may decrease the accuracy of dose calculation, especially when applying heterogeneity correction. 1,3 Structures with a high atomic number present within a field of view produce image artifacts that reduce image quality and create areas of misrepresented density information. These CT artifacts are unavoidable if the patient has dental fillings or metal hip prostheses and these structures are enclosed within or near the radiation treatment fields.…”

“…The use of IVICM during the acquisition of planning CT images is usually minimised, because it is possible that planning systems will misinterpret IVICM as high density tissue, leading to an underestimation of the actual delivered treatment dose. 4 Nonetheless, the use of IVICM with CTs for some specific tumour types is recommended to improve tumour volume definition as well as the enhancement of other regions of interest, 3 particularly for radical treatment.…”

“…1 There is conflicting evidence regarding the impact of IVICM on dosimetry. Ramm et al 3 (using phantoms containing materials to simulate the presence of contrast) showed that contrast agents had effects on dosimetry which were more pronounced as concentration and volume of the contrast media increased. These authors considered dose changes of 1-3% for patients displaying Hounsfield units of less than 500, in volumes of less than 5 cm to be acceptable.…”

“…These authors considered dose changes of 1-3% for patients displaying Hounsfield units of less than 500, in volumes of less than 5 cm to be acceptable. 3 However, Williams et al,…”