Medullary tumors affect thyroid cells that produce a hormone that does not contain iodine. Although these tumors grow slowly, they may be harder to control than papillary and follicular tumors. The cancer cells tend to spread to other parts of the body. Only about 5 percent of all thyroid cancers are medullary tumors. It is estimated that 1 out of 10 medullary thyroid cancers is hereditary.
Anaplastic tumors are the fastest growing thyroid tumors.
The cancer cells, which are extremely abnormal, spread rapidly to other parts of the body. Anaplastic tumors make up about 18 percent of all thyroid cancers and usually occur in people over 60 years of age. Causes Doctors do not know what causes most cases of thyroid cancer. However, scientists have observed that thyroid cancer affects women two to three times as often as men and occurs more frequently in whites than in blacks. One known risk factor is exposure to radiation during childhood.