The Impact of Gadolinium-based Contrast Agent for Carbon Ion Radiotherapy

Purpose The contrast agent used in the diagnostic department has high atomic numbers and might influence dose deposition in the particle therapy. In particular, the influence of gadolinium-based (Gd) contrast agent on range in carbon ion radiotherapy has not yet been evaluated. For this reason, we avoid carbon treatment and planning computed tomography (CT) acquisition on days when the contrast-enhanced magnetic resonance image (MRI) is performed. In this study, we evaluated the time required for this beam range effect to vanish by evaluating the temporal changes in the CT values after an enhanced MRI as well as the stopping power of Gd solution. Materials and methods Two types of diluted solutions with Gd contrast agent were used for comparing their transferred stopping power (TSP) and measured stopping power (MSP). The TSP was calculated with a CT value to stopping power ratio table that was created previously. Additionally, to evaluate in vivo attenuation, we measured the CT values in the renal pelvis from the CT images with and without contrast agent for 73 patients. Results The maximum difference between the TSP and MSP was 85%. The difference between the TSP after 4 hours and the TSP with non-enhanced cases was less than 1%. Moreover, the difference between the MSP after 1 hour and the MSP with non-enhanced cases was less than 0.1%. Conclusion It was found that the impact of Gd contrast agent can be neglected 1 hour after administration for carbon beam irradiation and 4 hours after for planning the CT image acquisition.