The effect of intravenous contrast on intensity-modulated radiation therapy dose calculations for head and neck cancer

Objective: The objective of this study was to determine if the use of intravenous contrast results in clinically important errors in intensity-modulated radiation therapy (IMRT) dose calculations for head and neck radiotherapy treatment planning. Materials and Methods: Nonionic, iodinated intravenous contrast (Iohexol) was administered during the treatment planning computed tomography (CT) scan of 5 patients with head and neck cancer of varying disease sites. The potential effect of intravenous contrast was studied by changing the density of the contrast-enhanced vessels. An inverse IMRT plan was generated from an unmanipulated "normal contrast" planning scan. We then applied the same planning parameters to a "no contrast" planning scan. The effect of intravenous contrast was quantified by calculating the percent change of dose in a variety of target and normal structures. To evaluate a worst-case scenario situation, this comparison was repeated by assigning the vessels the maximum density in our planning system ("maximum contrast" density plan). Results: Dose differences between a planning set of images using intravenous contrast and a set of images without contrast were less than 0.2% for all relevant target volumes and critical structures. A worst-case scenario in which normal contrast was overridden with "maximum contrast" led to small dose differences, generally less than 0.5%. Conclusions: Planning head and neck IMRT from CT scans that contain intravenous contrast does not result in clinically important errors in dose delivery.