The genetic DNA in the cell nucleus is thought to be the critical target for radiation-induced damage. Some radiation comes from natural sources, such as that from cosmic rays and radioactive substances in the earth's crust. But patients who receive daily radiation therapy treatments of 2 Gy directed to a small area of the body can be exposed to tens of gray over a period of weeks. Some of what we now know about the effects of radiation exposure was learned by studying patients treated with radiation in the past. These patients were found to have more leukemia and cancers of the lung, esophagus, bone, and other organs within the irradiated field than would be expected in a healthy population.
Second cancers following radiation treatments are rare but do occur.
There is a high risk of thyroid cancer many years after childhood exposure to radiation for therapy of noncancerous conditions of the head and neck. Exposure during childhood can be particularly damaging because rapidly growing cells may be more sensitive than slower-growing cells irradiated later in life. Before 1950, individuals with enlarged thymus glands were treated with intense radiation. Those who may have received radiation treatments for thymus conditions or other childhood problems, such as scalp ringworm or enlarged tonsils, should alert their physicians. A classic study of women who painted radium dials before 1930 showed high rates of bone sarcomas and head cancers.