In vivo retention of iodide by myeloma tumor xenografts expressing hNIS. Mice bearing myeloma tumor xenografts expressing hNIS were imaged serially starting one hour after injection of the isotope. Initial image acquisition was 3 minutes, but this increased to 5 minutes as the isotope decayed. After correction for the physical half-life of the isotope, acquisition time, and background activity, percentage iodide uptake in the tumors was calculated. There is rapid and high-level uptake of iodide by the tumors. Although most of it leaks out within the first few hours, low-level activity (4%-5%) remains up to 48 hours after injection.