The rising incidence of thyroid cancer over the past 3 decades has been attributed to incidentally discovered thyroid nodules, demographic variables, and/or environmental impacts. Additional known risk factors associated with an increased risk of thyroid cancer include family history of the disease, radiation exposure, Hashimoto’s thyroiditis, and elevated thyroid-stimulating hormone (TSH). Obesity, race, and socioeconomic factors also contribute to higher advanced thyroid cancer rates. Despite our increased understanding of thyroid cancer risk factors, collectively, they do not fully explain why the incidence rates are rising worldwide.
California has a very diverse population with a wide range of races and ethnicities, as well as a broad variety of international immigrants. Geographically, the state of California also has diverse topography and is comprised of large areas of coastline, mountainous regions, valleys, vast expanses of desert or arid land, as well as a plethora of agricultural areas scattered throughout the state. It also has one of the largest collections of decommissioned nuclear reactors in the country. Other than the known impact of ionizing radiation on the development of thyroid cancer, only a few studies have addressed other possible environmental exposures as contributing factors to the rise in thyroid cancer incidence.
This area of research has been difficult to study, due to the geographic mobility of many area residents and the difficulty in collecting exposure data. However, there are some studies that have emerged, suggesting exposure to environmental chemicals might increase the risk of thyroid cancer. A link between environmental toxins/pesticides and thyroid cancer has been suggested, but no study large enough to assess that implication has yet been conducted. High urinary levels of the metal tungsten have also been associated with elevated TSH levels, which in turn could increase one’s risk of thyroid cancer. Also, an individual’s likelihood of being exposed to any of these—or other—environmental risk factors could vary with geographical location.
In our tertiary referral center, we have noted an unusually high rate of advanced thyroid cancers presenting from across the state of California. To investigate the possibility of environmental impacts as a predisposition for thyroid cancer, we sought to identify counties in California with possible geographic clustering of advanced thyroid cancer cases. We recently published our findings in the Journal of Surgical Research
A total of 26,983 patients with newly diagnosed thyroid cancer from 1999 to 2008 were reviewed from the California Cancer Registry (CCR) and the Office of Statewide Health Planning and Development Registry. Established in 1947, the CCR is California’s statewide population-based cancer surveillance system and is one of the largest, most comprehensive cancer registries in the world. It captures information on every patient who is newly diagnosed with cancer in the state and follows his or her progress forward. It has collected detailed information on over 3.4 million cases of cancer, and more than 162,000 new cases are added annually.
Advanced thyroid cancer rates were calculated within each county—defined as those with distant metastatic stage, regional and/or distant metastatic stage (RM), as well as those with well-differentiated thyroid cancer (WDTC) diagnosed before age 30. National averages were taken from Surveillance Epidemiology and End Results (SEER) data. For our outcome, we used the stage of disease at presentation rather than the rising cancer incidence within each county. This was chosen because incidence rates of thyroid cancer are rising throughout the state, the country, and the world.
When we looked at our own counties, each one had an increased incidence over the course of the previous 10 years. The rate of increase was not different from what is already known for the rest of the country and in other parts of the world. Our interest was more about how delayed the cancers were being diagnosed, since that significantly impacts patient care, morbidity, and mortality.