Lack of influence of intravenous contrast on head and neck IMRT dose distributions

Abstract

Purpose. Intravenous (IV) contrast at the time of CT-Simulation facilitates radiotherapy contouring, but may introduce a discrepancy between planned and delivered dose due to density variation in blood vessels. Here, the effect of physiologic and non-physiologic extremes of IV contrast densities on intensity modulated radiotherapy (IMRT) plans for patients with head and neck cancer was investigated. Methods and materials. This planning study was conducted using IV contrast CT scans of ten patients with squamous cell cancer of the head and neck treated with IMRT. The target volumes and normal tissues, including the blood vessels of the head and neck, were contoured and IMRT plans were created according to RTOG Protocol 0022. The density within the blood vessels was then virtually altered to mimic non-contrast and extreme (bone and air) densities. The dose was then recalculated using the same IMRT plan. Plans obtained with and without density overrides were then compared. Results. The change in planning target volume (PTV) coverage for plans with and without IV contrast was minimal. The volume of the PTVs covered by the 93% and 100% isodoses changed on average by 0.57%. The minimum dose to PTVs varied by a maximum of 0.17 Gy. The maximum point dose to critical organs changed by a maximum of 0.12 Gy (brainstem). Non-physiologic extremes of density within blood vessels also resulted in minimal changes in tumor or normal tissue dosimetry. Conclusion. The use of IV contrast at time of CT-simulation does not significantly affect dose calculation in head and neck IMRT plans.