Effect of contrast agents used in computed tomography simulation on inhomogeneity dose calculations for a three dimensional treatment planning system

The purpose of this study is to investigate the relationship between heterogeneity corrected dose calculations in the presence of iodine based contrast agents or with heterogeneity corrections and density assignment to a contrast containing organ. In addition, the study was used to assess a density assignment method to minimize any effects on dose calculations attributed to these contrast agents and assess how dose calculations using CT scans with contrast agents relates to the CT scanning kilo voltage technique. A two part study was conducted. Part one utilized a phantom constructed with a 236 ml container filled with varied concentrations of 60% Hypaque contrast agent (Diatrizoate Meglumine Injection, USP). The contrast filled container was mounted in an 8 cup bowl filled with saline solution. The phantom containing contrast was scanned while varying the independent variables of contrast concentration and scanning kilo voltage settings. Dose calculations were performed for computing monitor units necessary to deliver the prescribed dose. Second portion of the study utilized more lifelike conditions using contrast by selection of actual clinical cases. Data were derived from 30 case studies determined to have met the following characteristics: prostate cancer patients administered 4 cc of contrast mixed with 40 cc of saline administered using a retrograde technique to delineate the bladder on CT images. Cases were scanned using the standard 120 kVp technique and 3 mm slice thickness. This study utilizes a three dimensional treatment planning system employing an Anisotropic Analytical Algorithm (AAA) for the calculations. Dose Volume Histogram data, dose differences and Distance to Agreement (DTA) were evaluated in comparing the heterogeneity corrected and density overridden contrast containing organ calculations. This study concluded an increased density due to iodinated contrast agents is associated with an increased dose per monitor unit calculation for minimum dose to the Clinical Tumor Volume (CTV). A decreased scanning kilo voltage is associated with an increased dose per monitor unit calculation as well.Normal tissue density assignment to contrast containing organs is associated with an increase in calculated CTV dose. Though the findings were statistically significant the largest effects were observed in the phantom tests, clinical significance is small when utilizing low levels of contrast concentration. Any dose effects become clinically insignificant as the number of treatment fields is increased and the contrast concentration is decreased. Density assignment to the bladder for prostate treatment plans is a valuable option for increasing accuracy of the calculation when contrast HU's are high however, contrast concentrations normally used in the clinic will not significantly change the treatment plan, CTV dose or bladder dose by more than 1% if not employed.