Epidemiology is the study of factors that affect the health and illness of the population. It can serve as a foundation for identifying risk factors for diseases, which can be used to develop a strategy of intervention. It can also be used to identify optimum treatment practices. Dr. Yawei Zhang, Professor of Epidemiology and Public Health at Yale School of Medicine, studies the relationship between environmental risk factors and cancer. She has studied the relationship between hair dye use and non-Hodgkin lymphoma, and recently received funding from the American Cancer Society to study the link between radiation and thyroid cancer.
“Non-Hodgkin lymphoma is a very important, but understudied, disease in terms of what causes it. The Yale Cancer Center Prevention and Control Research Program has done a pivotal study in the epidemiology of non-Hodgkin lymphoma,” said Dr. Zhang. “The incidence of non-Hodgkin lymphoma has been increasing during the past several decades with unknown reason. Based on findings from an epidemiology study that the National Cancer Institute conducted in Iowa and Nebraska, it may be related to the personal use of hair dye.”
In order to test this theory in the Connecticut population, Dr. Zhang and her colleagues conducted a population-based case-control study among Connecticut women. They recruited 601 non-Hodgkin lymphoma cases and 719 population controls. “We administered a questionnaire and collected blood samples. A lot of epidemiologic studies are based around DNA and it is crucial to look at the gene-environment interaction to identify who is susceptible to certain environmental carcinogens, because only some people get cancer and we need to figure out what causes it to develop in some, but not others,” said Dr. Zhang.
From the study they did find increased risk for women who started using hair dye before 1980, a period when hair dye contained carcinogens in their ingredients. Women who used darker permanent hair coloring products for more than 25 years showed the highest increased risk. They did not, however, find an increased risk for women who started using hair dye in 1980. According to Dr. Zhang, this could reflect the change in hair dye formula contents over the past two decades. To further confirm the findings, she conducted a pooled analysis using data from the International Lymphoma Epidemiology Consortium including 4,461 non-Hodgkin lymphoma cases and 5,799 controls. Results from this large pooled project indicated that an increased risk associated with hair dye use was mainly seen for follicular lymphoma, a major subtype of non-Hodgkin lymphoma, and this risk was not limited to women who started using hair dye before 1980.
Over the last 40 years, thyroid cancer has also increased, more than two-fold. One of the major arguments for this increase is over diagnosis of the disease. However, almost every single country has an increased incidence of thyroid cancer. Dr. Zhang explained that there are two environmental factors that are suspected as the cause for the increase. One is that current diagnostic x-ray use may explain the increasing trends, another is centered on a chemical used in flame retardants called PBDE.
“There must be some environmental changes that caused this increase in thyroid cancer and we know that radiation is the major environmental risk factor. During the 1950s and 1960s there was a lot of radiation treatment for the head and neck that might explain some of the increase around the early trends, but it does not explain the later trends,” said Dr. Zhang. Dr. Zhang explained that the Chernobyl incident resulted in the first discovered connection between radiation exposure and thyroid cancer.
Based on some of their work demonstrated with animal models, Dr. Zhang and her colleagues received a grant from the American Cancer Society to look at radiation exposure, especially the diagnostic x-ray exposure and DNA repair capacity in relation to thyroid cancer because radiation mainly affects your DNA repair capacity and can cause double strand breaks. They hope to use the same population in the future to get additional funding to test the PBDE hypothesis as well.
Dr. Zhang believes that cancer epidemiology will gain awareness over the next decade. “For every single disease, there are not purely genetic and purely environmental causes; there must be a gene-environment interaction. There are certain genetic susceptibilities and we encounter certain environmental carcinogens, so it is important to look at our ‘fingerprints’ and try to predict our chances of a future diagnosis of cancer,” said Dr. Zhang.