Thyroid cancers, radiation exposure

The nuclear explosions that devastated Hiroshima and Nagasaki killed 100,000 to 200,000 people instantaneously. Probably an equal number died later, victims of the radiation released in those explosions. Millions of people were exposed to the radioactivity released by the accident at the Chernobyl nuclear power plant. The full health effects of that accident may never be known, but 31 people died of radiation sickness within a few weeks of the accident, and more than 2000 people have developed thyroid cancer through exposure to radioactive iodine released in the accident. Even low levels of radiation can cause health problems. For this reason, workers in facilities that use radioisotopes monitor their exposure to radiation continually, and they must be rotated to other duties if their total exposure exceeds prescribed levels. 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

After the nuclear explosion at Chernobyl in 1986, Anatoly and other professors and physicians created a foundation, For the Children of Chernobyl. Their goal was to send children abroad for the summers for a reprieve from radiation exposure which impairs their immune systems and has resulted in unprecedented levels of thyroid cancer in children and adults. The first host country to respond to their call for help was India. Before long, the foundation was sending 30,000 children every summer to host families and programs in many countries, including Germany, England, Japan, the U.S., Spain, Italy and France. 

Rubino, C., Cailleux, A., DeVathaire, R, Schlumberger, M. (2002). Thyroid cancer after radiation exposure. European Journal of Cancer, 38(5), 645-647. 

Within the following few weeks 28 of the 32 acute deaths of exposed employees were judged due to radiation exposure. Thyroid cancer in children under 18 rose from an incidence of 0.5 of 100,000 (1986-1988 baseline) to more than ten times that level (5-8 per 100,000) in Belarus (1993-2002). Increases were just as consistent, but of less magnitude for Ukraine, going from 0.2 per 100,000 from 1986 to 1988 (baseline) to 5-10 times that level (1 to 2.2 per 100,000) from 1993 to 2002. There is little doubt that Chernobyl radiation caused thyroid cancer. In 2,000 there were 4,000 cases of thyroid cancer in children under 18 drinking milk contaminated with in 2002 there were 12 deaths related to these exposures. 

Cancer is the major effect of low radiation doses expected from exposure to radioactive contamination. Laboratory studies have shown that a-, /S-, and y-radiation can produce cancer in virtually every tissue type and organ in animals that have been studied (ATSDR, 2001). Cancers observed in humans after exposure to radioactive contamination or ionizing radiation include cancers of the lungs, female breast, bone, thyroid, and skin. Different kinds of cancers have different latency periods leukemia can appear within 2yr after exposure, while cancers of the breast, lungs, stomach, and thyroid have latency periods greater than 20 yr. Besides cancer, there is little evidence of other human health effects from low-level radiation exposure (ATSDR, 2001 Harley, 2001). 

The situation in relation to thyroid effects is serious. Up to the end of 1995, there were more than 800 cases of thyroid cancer reported in children, mainly in Belarus. Thyroid cancer may be induced by causes other than radiation, but all these cases seem likely to be associated with radiation exposure due to the accident. They represent a dramatic increase in the normal incidence of this rare type of cancer and the increase seems not to persist among children born after 1986. Thyroid cancer is usually non-fatal with early diagnosis, treatment and attention. At the time of the Chernobyl Conference, three of the children affected had already died. The prospects cannot be precisely predicted the high incidence is expected to continue for some time and the number of reported cases may be in the thousands the mortality will depend very much on the quality and intensity of the treatment given to the affected children. 

Perhaps the best known use of potassium iodide today is as a treatment for radiation exposure. When a nuclear bomb explodes or a nuclear accident occurs, one of the most dangerous products released to the environment is a radioactive isotope known as iodine-131. Iodine-131 enters the human body and travels to the thyroid, where it attacks cells and tissues, eventually resulting in thyroid cancer. Experts recommend that people exposed to radiation take potassium iodide as a protection against this hazard. The potassium iodide saturates... 

Radioiodine plays an important role in the diagnosis and treatment of various thyroid disorders. Treatment of thyroid carcinoma and hyperthyroidism with I-pharmaceuticals has been practiced for years, but other isotopes, i.e., I, >231, 1241 and are also produced and used in various medical apphcations. Radioiodine concentrated by the thyroid in large amounts can cause cell death, primarily because of 3ils beta radiation. Large doses of 3il are, therefore, given to treat patients with hyperthyroidism. In contrast, low-dose exposure damage does not kill thyroid cells, but can induce radiation damage and mutations, which can result in thyroid cancer. 

The thyroid gland is unique in its developmental sensitivity to malignancy following radiation exposure. Individuals older than 20 years of age do not have an increased risk of thyroid cancer when exposed to low-level thyroid irradiation (Boice, 2005, 2006 Ron et al., 1995). Yet, when individuals are less than 20 years of age at the time of low-level thyroid irradiation, the thyroid cancer risks increases (Boice, 2005, 2006 Ron et al., 1995). 

In addition to age, the radiation dose plays a major role in causing cancer (Dolphin, 1968 Ron et al., 1995 Boice, 2005, 2006). The risk of thyroid cancer and thyroid nodules is highest with exposure to low or moderate levels of external radiation (0.1-25 Gy), and not with the considerably higher doses used internally to treat Graves disease (>150Gy) (Dolphin, 1968 Ron etal., 1995 Boice, 2005, 2006 Sigurdson et al., 2005). 

For children less than 5 years of age, we consider antithyroid medications as a first-line therapy. Radioactive iodine has also been successfully used in this age group without an apparent increase in cancer rates. Yet, it may be best to defer radioactive iodine therapy because of the possible increased risks of thyroid cancer after radiation exposure in very young children in the event that any thyroid tissue remains after radioactive iodine therapy, and to avoid the low level whole body irradiation associated with radioactive iodine. 

When administered as a major amount concentrates in the thyroid gland, although to a lesser extent in differentiated thyroid cancer compared with normal thyroid tissue. The short range of the emitted beta particles leads to cell damage and cell death. The emitted radiation, however, can be harmful to other organs of the patient and has the potential to induce cancer, especially with repeated treatments and high cumulative activities. Other people in the vicinity of the patient may be exposed to external radiation and contamination. For children, thyroid cancer after radiation exposure appears to be a significant risk, as has been documented after the accident in Chernobyl. 

Risk of Exposure of Hospital Staff The most important exposure of hospital staff is caused by external radiation. The estimated dose received by nursing staff is determined by the mobifity of the patient and the degree of nursing care required. Barrington etal. (1996) estimated the cumulative annual dose to nursing staff in the first 7 days after treatment of thyroid cancer patients with l. 

Radiation exposure has been associated with most forms of cancer in many organs, such as the lung, breast, and thyroid gland. If the cell is only modified by the radiation damage, the damage is usually repaired. In some conditions, the repair mechanism may not be perfect, and the modification will be transmitted to daughter cells. This may eventually lead to cancer in the tissue of the exposed individual. Radiation-induced cancer may manifest itself decades after the etqiosure and does not differ from cancers that arise spontaneously or are attributable to other factors. 

There is a considerable latent period between radiation exposure and the appearance of cancer. For most cancers in adults, the latent period is at least 10 years, or even longer. The shortest latent period is for leukemia and thyroid cancer (3 to 5 years The appearance of radiation-induced cancers follows additive or multiplicative models of prediction with absolute or relative risks as main parameters. Assessment of the risk coefficients is based on the follow-up of exposed persons through epidemiological studies. 

The plume will travel downwind and the concentration of radioactive materials will tend to decrease as it travels further from the plant. As the concentration of radioactive materials in the plume decreases, the dose rate to the affected population will also decrease. Thus, those who are further away from the plant will generally be at less risk of deterministic (early) health effects. While the exposures further from the plant are small, they all add to the chance of getting cancer (stochastic effects). Since the total amount of human exposure is larger further from the plant (large number of people exposed to small amounts of radiation), this is where most cancers will occur. Following the Chernobyl release the vast majority of the excess thyroid cancers caused by the accident occurred between 50 and 350 km from the plant. 

The IAEA recognized that the number of deaths attributable to the Chernobyl accident has been of paramount interest to all concerned - the general public, scientists, the mass media, and pohticians. The IAEA noted that claims had been made that tens or even hundreds of thousands of persons have died as a result of the accident, and stated that these claims were exaggerated the total number of people that could have died or could die in the future due to Chernobyl-originated exposure over the lifetime of emergency workers and residents of most contaminated areas was estimated to be around 4000. This included some 50 emergency workers who died of acute radiation syndrome in 1986 and other causes in later years, 9 children who died of thyroid cancer, and an estimated 3940 people that could die from cancer contracted as a result of radiation exposure. 

---