Vandetanib, the drug approved today, failed in non-small cell lung cancer and in bladder cancer before it proved effective in medullary thyroid cancer.

FORBES: AstraZeneca's Victory Highlights Difficulty Of Cancer Drug Development

Medullary thyroid cancer involves specific types of cells that are found in the thyroid gland and can occur spontaneously, or be part of a genetic syndrome.

FORBES: AstraZeneca's Victory Highlights Difficulty Of Cancer Drug Development

Nothing is likely to answer the question definitely short of approving the drug, letting millions use it, and waiting to see whether more people get medullary thyroid cancer.

FORBES: Drug Safety