Soon after buying her first home on Cape Cod in 1978, Cheryl Osimo noticed a green truck pulling up to a neighboring lot. A man stepped out, his face obscured behind a thick plastic helmet and his body encased in heavy coveralls. As he trudged through the woods to the small pond behind Osimo’s house, he reminded her of an astronaut walking on the moon. Through the gaps in the trees she could see him spraying for mosquitoes—a ritual, she later learned, that occurred several times a year.

“To this day,” Osimo says, “I’m dismayed about my own ignorance. Why didn’t I think about how his spraying would affect us downwind? Why didn’t I realize that if he wore all that protective gear, I should be worried about my children playing in the sandbox? And why wasn’t I concerned about those chemicals drifting into our home?”

Just over a decade later, when Osimo was diagnosed with breast cancer at the age of forty, she struggled to understand why she had developed the disease. After all, she had no family history and no known risk factors.

“People placed such an emphasis on looking good to feel good,” she says. “Someone even called to help me with my eyebrows because I’d lost them to chemo. I felt frustrated, because I wasn’t hearing anything about what had caused my cancer or how I could prevent my children from getting sick. I never want them to go through chemo and radiation; because of my cancer, I feel like they already have.”

Two years after her diagnosis, Osimo learned that nine of fifteen Cape Cod towns had some of the highest breast cancer rates in the commonwealth. Already a member of the Massachusetts Breast Cancer Coalition, she helped mobilize a response—and ended up helping to found Silent Spring Institute to investigate possible environmental links to breast cancer. As Silent Spring’s Cape Cod coordinator, Osimo now serves as a linchpin in the Institute’s relationship with the community.

And that relationship is key to the Institute’s work. Soon after its founding, Silent Spring received $3 million from the Massachusetts Department of Public Health to launch the Cape Cod Breast Cancer and Environment Study. Ever since then, Institute scientists have been working with Cape Cod women to map the women’s geographic susceptibility to breast cancer.

In the first three years of the study, the researchers amassed critical data through painstaking fieldwork. They created maps of water supply pipes, pored over records of wide-area pesticide applications, and traced septic systems. They found that now-banned persistent organochlorine pesticides were typically used on Cape Cod from 1948 to the mid-1970s, with less persistent—and perhaps less toxic—compounds used in more recent years. By superimposing such data in a computerized geographic information system (GIS), the investigators created a powerful diagnostic tool for assessing environmental risks on the Cape.

“The GIS replaces paper maps and transparent overlays with a sophisticated mix of computer hardware, software, and expertise, all focused on being able to visualize and analyze spatial data,” says Julia Brody, PhD, executive director of the Institute. “Beyond simple mapping, the GIS allows us to integrate layer upon layer of different kinds of information. Gaining insight into the spatial and temporal relationships between these layers can help us develop and evaluate causal hypotheses, which in turn moves us closer to our goal of disease prevention.”

One critical layer of information has come from interviews the researchers conducted with 1,121 Cape Cod women who had been diagnosed with breast cancer and 992 comparison women of similar ages. The interviews captured details about each woman’s breast cancer risk factors, including her age, family history of the disease, and childbearing experience. Researchers also mapped the women’s Cape Cod addresses dating back to 1948, when DDT was first used on the Cape, and linked interview responses to GIS data that reconstructed the history of pesticide use.

The researchers then developed a new computer method, the Spatial Proximity Tool, which allowed them to estimate women’s pesticide exposure at each address for each year based on such factors as the distance to spray areas and wind direction. They were hampered to a degree, though, by a lack of information about women’s exposures during years they lived off Cape Cod, a lack of follow-up with girls and women who moved away, and town and private pesticide sprayings that went unrecorded.

After controlling for established risk factors, the researchers discovered that women who have lived longer on the Cape are at higher risk for breast cancer than new arrivals. Women who lived on Cape Cod for 25 to 29 years were at highest risk—72 percent higher risk than women who lived there fewer than five years.

The results, published in the February issue of Annals of Epidemiology, suggest that environmental factors are playing a role on Cape Cod—and that finding preventable causes of breast cancer may be possible. “Part of the strength in these results,” Brody says, “lies in their use of women’s addresses dating back years before their breast cancer was diagnosed, providing information about the time when the tumor developed, rather than just one address at the time the cancer was diagnosed.”

Other recent results from the Cape Cod Study, published in Environmental Health Perspectives in June, found no consistent or strong pattern of association between breast cancer and pesticides used for tree pests, cranberry bogs, other agriculture, mosquito control on wetlands, golf courses, or rights of way management.

Yet the researchers did observe some suggestive associations. They found that breast cancer risk was somewhat elevated for women who lived in or near areas treated for tree pests between 1948 and 1995, near cranberry bogs between 1948 and the mid-1970s, and near agricultural land since the mid-1970s. As expected, risk was higher for women in residences with no tree buffer to protect them from pesticide drift.

“These findings warrant follow-up,” Brody says, “especially because women continue to be exposed to the current-use pesticides, which haven’t been studied much.” And Brody remains undaunted about the painstaking nature of the work. “We have to uncover layers and layers of clues, just as if we were peeling the layers of an onion,” she says. “We must, for example, estimate a woman’s exposure to chemicals years—even decades—before her tumor first appeared. It is unlikely that any one study will bring dramatic breakthroughs. Rather, knowledge will build incrementally, from many studies using multiple methods.”

Brody adds that the data and tools developed during the Cape Cod Study represent a rich resource for studying a range of health and environmental questions. The Spatial Proximity Tool, for example, while demonstrated in this study for historical analysis, can also be used to model future hypothetical events. “Our ultimate goal,” Brody says, “is to discover risk factors for breast cancer that we can change.”

Cheryl Osimo shares Brody’s determination.

“When I was first diagnosed,” she says, “one in nine women could be expected to develop breast cancer in their lifetimes. Just a year later I was attending a rally with my seven-year-old daughter when we noticed some women inking out the ‘nine’ on T-shirts and writing in ‘eight.’ And recently the odds have risen to one in seven. If we don’t focus our energies on prevention, the risk will continue to climb.”