A study of nearly 133,500 female teachers in California begun in 1995 shows that these educators experienced a 51 percent higher rate of breast cancer than comparable California women. They also had a 72 percent greater risk of endometrial cancer, according to investigators from the Keck School of Medicine of USC, UC Irvine, the Northern California Cancer Center and the state Department of Health Services.
"Clearly, teachers face a higher risk of many cancers. These women most likely have in common certain risk factors that contribute to their increased risk. As we gather more information on our study participants over the next few years, we should gain insight into the causes of these cancers," says Leslie Bernstein, Ph.D., professor of preventive medicine and AFLAC Chair in Cancer Research at the Keck School of Medicine and lead author of the study.
The study is the first to extensively examine cancer among schoolteachers. It reports on cancer incidence from 1995 to 1998 in 133,479 current and former public school teachers or administrators participating in the California State Teachers Retirement System.
Researchers expect that lessons learned about cancer within the group of participating teachers will increase understanding of the roots of cancer among all women.
About 87 percent of study participants are non-Hispanic white, though substantial numbers of African Americans, Hispanics, Asians and Pacific Islanders are involved.
Researchers found the following for teachers:
- 28 percent greater risk of ovarian cancer
- 59 percent greater risk of melanoma
- 47 percent greater risk of lymphoma-mostly non-Hodgkin's lymphoma
- 28 percent greater risk of leukemia-mostly chronic forms
- 34 percent lower risk of lung cancer
- 47 percent lower risk of cervical cancer
"What surprised me about these results was the magnitude of the excess risk, particularly for breast cancer," says Ronald K. Ross, M.D., professor and Flora L. Thornton Chair in Preventive Medicine at the Keck School. "There are some serious issues here that need to be understood and addressed. This high risk is particularly alarming given the large number of active and retired female teachers residing in California."
Investigators believe teachers tend to share certain lifestyle and environmental factors that may help protect them from cancer or increase risk.
They found these characteristics among participating teachers and compared them to similar women throughout California:
|Average age at first menstruation||12.5 years||12.9 years|
|Never pregnant||21 percent||14 percent|
|Average age at first pregnancy||26.4 years||23.7 years|
|Average number of live births||2.3||2.7|
|Has had a hysterectomy||24 percent||29 percent|
|Body mass index less than 25||61 percent||53 percent|
|Has never smoked cigarettes||67 percent||55 percent|
"Over the next few years, we will use the information provided by teachers to better understand the causes of certain cancers and other health conditions among women," says Dennis Deapen, Dr. P.H., professor of clinical preventive medicine at the Keck School and director of the Los Angeles County Cancer Surveillance Program.
No one knows exactly why, but women with more education and income are at higher risk for breast cancer. This is at least partly because women who delay having their first child and have fewer children-until after college or starting a career, for example-are at increased risk.
Breast, ovarian and endometrial cancers share some of these reproductive risk factors. Teachers in the study tended to have their first baby later and had fewer children in total. Teachers also had high rates of use of hormone replacement therapy, which may be linked to breast cancer. These factors, however, do not appear to entirely explain the excess risk, according to Ross.
Meanwhile, cervical cancer, a disease occurring less frequently among teachers, can be prevented through Pap testing-and a substantial 91 percent of participants reported having a Pap test sometime within the previous two years. Smoking was low among teachers, which might explain the reduced lung cancer incidence among them.
In the coming years, the investigators will send out additional questionnaires and look closely at factors such as family history, diet and environment. Bernstein, for one, will examine relationships between physical activity and breast, endometrial and ovarian cancer risk.
The investigators hope to uncover risk factors behind the higher rates of melanoma, lymphoma and leukemia as well. "We expect to see many more initiatives coming out of this study," Bernstein says.
Teachers have long been suspected to be at high risk for breast cancer. This study grew out of reports of excess incidence of breast and other cancers among women in California school systems.
Although no one can prevent cancers from occurring in the population, people can take steps to lower their risks. General recommendations include avoiding smoking, eating a healthy diet and exercising. More information on cancer risk reduction is available from the American Cancer Society at www.cancer.org and the National Cancer Institute at 1-800-4-CANCER.
The study was supported by the California Breast Cancer Act of 1993 (tobacco tax), the California Department of Health Services and the National Cancer Institute.
Leslie Bernstein, Mark Allen, Hoda Anton-Culver, Dennis Deapen, Pamela L. Horn-Ross, David Peel, Richard Pinder, Peggy Reynolds, Jane Sullivan-Halley, Dee West, William Wright, Al Ziogas and Ronald K. Ross, "High breast cancer incidence rates among California teachers: results from the California Teachers Study (United States). "Cancer Causes and Control. September 2002, Vol. 13, No. 7.