An Efficacy Study Comparing ZD6474 to Placebo in Medullary Thyroid Cancer

The purpose of this study is to learn how hereditary or sporadic medullary thyroid cancer patients, treated with ZD6474, react to the drug, what happens to ZD6474 in the human body, about the side effects of ZD6474, and if ZD6474 can decrease or prevent the growth of tumors.