Summary. The rate of thyroid cancer, which is rising faster than any other cancer, is highest in Pennsylvania. Within the state, the highest rates are in the eastern counties closest to and east (downwind) of four nuclear power plants. Because radioactive iodine found only in nuclear weapons and reactors is known to cause thyroid cancer, it would be helpful to conduct closer examination of reactor emissions and the disease.
Background. Thyroid cancer is a relatively rare type of malignancy, accounting for about 1-2% of all new cancers in the U.S. It is one of the least deadly cancers, with a five year survival rate over 95%. Not much is known about the causes of thyroid cancer; the most recognized risk factor is exposure to ionizing radiation, specifically to radioactive iodine.
Established cancer registries in five states and four cities (9% of the U.S. population) show incidence of thyroid cancer changed little in from 1973-1980. From 1980-2003, incidence more than doubled from 4.3 to 9.4 cases/100,000 (adjusted to the 2000 U.S. population). Rates doubled for males, females, whites and blacks. Thyroid cancer is the fastest-rising cancer in the country, with no explanation for this trend.
A recent article in the journal JAMA suggested that better diagnosis accounts for the increase. This theory is questionable, as most thyroid cancers are found through routine physical examinations. No data exists that physicians are more proficient in examining patients for thyroid cancer, or provide more physical exams than a generation ago.
Geographic Patterns - by State. With cancer registries now operating in all states, the U.S. Centers for Disease Control and Prevention has assembled cancer incidence data for the years 1999-2002, for 38 states and the District of Columbia. Pennsylvania had the highest thyroid cancer incidence, at 10.61 cases per 100,000 persons, adjusted to the 2000 U.S. standard (5436 cases). The state with the lowest rate was North Carolina (4.89, 1207 cases). Table 1 shows rates for the states with the five highest and five lowest rates.
The differences between the states with the highest and lowest rates are substantial, for no apparent reason. Pennsylvania is of particular interest, not just because it has the highest rate, but because it is the 6th most populated in the U.S (see Table 2). Its high rates are consistent among all years, genders, and races (for 38 states and District of Columbia).
Geographic Patterns - by Pennsylvania County. The next step in a geographic analysis of thyroid cancer patterns is to examine rates by region or county. Such an analysis is feasible for Pennsylvania, as the state makes annual age-specific cancer cases for each county from 1990-2003 available on the internet. The 2000 U.S. census age-specific population allows age-adjusted incidence rates for each county to be calculated.
Average annual thyroid cancer rates for 1997-2003, adjusted to the 2000 standard U.S. population, were calculated for each of the 67 Pennsylvania counties. The state rate was 9.89 cases per 100,000 (8871 cases), with a wide range in rates by county (Table 3).
A remarkable pattern emerges from the county-specific analysis of thyroid cancer rates in Pennsylvania. Of the 14 counties with the highest rates, 13 are located in the eastern third of the state (Clinton is the exception). There are no apparent socioeconomic factors distinct to this part of the state. Some counties are densely populated, while others are rural. The percentages of poverty and minorities vary as well. Even if there were a clear-cut pattern, none of these factors suggest a high risk for thyroid cancer.
Thyroid Cancer in Pennsylvania and Nuclear Reactors. The risk factor most commonly associated with thyroid cancer is exposure to radioactive iodine. There are various forms of iodine; I-129 decays slowly (half life of 16 million years) while I-131 decays quickly (half life of 8 days). When iodine enters the body, it seeks out the thyroid gland, where it kills and injures cells, leading to cancer and other disorders.
Radioactive iodine is only created when atomic bombs explode or when nuclear reactors operate. Above-ground atomic testing ended in 1963 and all testing ended in 1992. Nuclear reactors have operated in Pennsylvania since 1957. There are five plants with 12 reactors, 9 of which are still operating (Table 4). The state has more reactors than any except Illinois. All plants except Beaver Valley are in the eastern part of the state.
Except for Clinton County, each of the 14 counties with the highest incidence rates from 1997-2003 are situated directly to the east, northeast, or southeast of a nuclear plant (see Map 1). This finding raises the theory that thyroid cancer risk has been raised by exposure to radioactive iodine, which is routinely released as airborne particles from each plant. Most iodine is propelled by prevailing winds, which blow from the northwest in colder months and the southwest in the warmer months.
Iodine enters human bodies through breathing and the food chain. One means to transport iodine into the body is water; east of reactors, municipal drinking water is typically obtained from local sources. Another vector is milk, which is often not produced locally, but transferred from dairy farms. Much of the milk consumed in eastern Pennsylvania is produced on farms in Lancaster and York Counties, which lie in the midst of the Limerick, Peach Bottom, and Three Mile Island nuclear plants.
There is often a lag of a decade or more between radiation exposure and onset of cancer. Seven of the 12 reactors in the state were operating by 1978, and all reactors had started by 1989. Thus, several decades have elapsed since reactors began emitting radioactive iodine into the environment, making the reactor-thyroid cancer link plausible.
The one Pennsylvania nuclear plant not in the eastern part of the state is Beaver Valley. The Beaver county thyroid cancer rate is not one of the highest in the state, but exceeds all but one of the 16 most western counties in the state (Table 5). The only western county with a higher rate is Lawrence, which adjoins Beaver. Typically, thyroid cancer incidence is low in the area; Armstrong, Crawford, Forest, Greene, and Venango counties are among the lowest seven in the state.
Implications. There is considerable state-by-state variation in the incidence of thyroid cancer, fastest-rising cancer in the U.S. The rate in some states is more than double that of others. In Pennsylvania, the state with the highest rate, there is considerable variation by county. Rates are highest in the counties in the eastern part of the state, each of which lies east of a nuclear power plant. The release of radioactive iodine into the atmosphere from these plants raises the possibility that these emissions are driving up thyroid cancer rates. More detailed study should be undertaken to better understand this relationship.
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