Vandetanib, the drug approved today, failed in non-small cell lung cancer and in bladder cancer before it proved effective in medullary thyroid cancer.
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.
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.
The problem is that medullary cancer is so rare and develops so slowly over decades, that there is be no easy way to rule out the possibility that the drugs might boost cancer risk decades after people start taking them.