The drug used in this trial is a modified version of the measles virus used to vaccinate children. The virus has been altered by having an extra gene (piece of DNA) added to it to allow a protein called NIS to be inserted into it. NIS is normally found in the thyroid gland ( a small gland in the neck) and helps the body concentrate iodine. Having this additional gene will make it possible to track where the virus goes in the body.