Internal radiation therapy of liver tumors: qualitative and quantitative magnetic resonance imaging of the biodistribution of holmium-loaded microspheres in animal models.

In internal radiation therapy of unresectable liver tumors, microspheres containing a radionuclide are injected in the hepatic artery to achieve a preferential deposition of microspheres in the lesions. In this study, MR imaging techniques for qualitative and quantitative assessment of the biodistribution of holmium-loaded microspheres (HoMS) were investigated for their use in selective internal radiation therapy of liver tumors. To achieve this goal, the relaxivity of HoMS was first investigated in gel experiments. The resultant calibration curve was subsequently employed to quantify the biodistribution of HoMS administered to 13 excised rabbit livers and to the livers of 3 live rabbits with an implanted tumor. Finally, the feasibility of MR imaging of the biodistribution during treatment of a large animal was investigated by MR imaging of hepatic administration of HoMS to a live pig. Overall, the study showed that MRI can clearly depict the biodistribution of HoMS, but that quantification by means of the gel calibration curve yields an underestimation that increases for higher amounts of HoMS. The observed underestimation is tentatively attributed to accumulations of HoMS in larger liver vessels. The exploratory quantification experiments suggest the feasibility of MR dosimetry. Copyright 2004 Wiley-Liss, Inc.