NEWS

From Crayons to Computers: Mapping Cancer Moves On

Katherine Arnold

Anyone who has ever taken an epidemiology course has a vivid flash upon hearing the phrase "the map of the Broad Street pump." John Snow’s 1854 graphical representation of cholera cases in central London ultimately tipped him off to the source of an epidemic that claimed more than 500 lives. This epidemiologic classic was the first to show the advantages of mapping disease.



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John Snow’s map of cholera deaths, originally published in 1855 in his book On the Mode of Communication of Cholera, led Snow to propose that cholera is a water-borne illness.

 
Some 120 years later, Thomas J. Mason, Ph.D., insisted to his superiors at the National Cancer Institute that maps of cancer mortality could reveal patterns that were previously unknown. His colleagues were unconvinced, so Mason and a student rolled up their sleeves and began coloring huge U.S. maps by hand.

With maps of stomach cancer, bladder cancer, and melanoma mortality as his ammunition, Mason pleaded his case one last time. Eyes widened as the maps were unveiled, and he was urged to move forward with the project as fast as he could. Twenty-five years later, his techniques have been refined, reformed, and copied around the globe.

"When Tom Mason first proposed doing county maps of cancer mortality, people couldn’t see how it could be a useful tool," said Linda Williams Pickle, Ph.D., of NCI’s Division of Cancer Control and Population Sciences. "But even people who don’t understand statistics can understand a map."

The Atlas of Cancer Mortality for U.S. Counties: 1950-1969, published in 1975, represented the first effort to map cancer mortality on the county level. Several research projects have grown from its findings, dozens of countries have copied its style, and new efforts to map cancer all have the same goal as the original atlas: to lead to a better understanding of local causes of malignancy.

A Tale of Two Countries

One of the most striking patterns to emerge from the first cancer atlas was a high mortality rate from oral and pharyngeal cancers among women in the southeast United States. Curious about the pattern, researchers conducted a case-control study in North Carolina—one of the states that had a high mortality rate for those cancers.Go



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This map published in 1975 shows the high concentration of deaths from oral and pharyngeal cancers among women in the southeast United States. Black indicates mortality rates are in the highest decile, and white indicates that rates are significantly lower than the U.S. average.

 
What they found was a high prevalence of smokeless tobacco use among women. Smokeless tobacco use was associated with a fourfold increased risk of oral and pharyngeal cancers; the risk jumped to fiftyfold for cancers of the gum and buccal mucosa.

Some 8,000 miles away, researchers in China in 1979 emulated the U.S. mortality atlas. The Chinese atlas revealed, among other things, a high mortality rate from esophageal cancers in a province in northern China.

Investigation into the region found something unusual: The chickens in the region frequently died from a condition similar to esophageal cancer. It seemed quite possible that lifestyle factors unique to that area could explain the high mortality rate in humans—but not the chickens.

Even more unusual was that the group of 50,000 residents had been relocated in the 1960s to a region 360 miles away because a reservior was built on their previous land. They acquired new livestock in their new land, but the same mortality patterns emerged within 2 years in both man and beast.

Further study revealed that pickled vegetables were a common staple in the diet of the locals, and at the end of the night table scraps were often thrown to the birds. While there are several risk factors for esophageal cancer, the mutagenicity of pickled vegetables seemed the likely link between cancers in chickens and their owners.

Hundreds more studies were sparked by the mortality atlas in the United States and in other countries that followed with their own atlases, including most European countries and the Soviet Union.

"When we went into the field to do a study, we came to a much better understanding of why these cancers are markedly elevated in a particular place," said Mason, who is now professor and chairman of the Department of Epidemiology and Biostatistics at the University of South Florida, Tampa.

Improvements

The first cancer atlas and the subsequent publications function on three levels: Researchers can determine a rate in any one place, note clusters and trends in cancer mortality, and compare different maps, said Pickle, who was the lead author on three mortality atlases—two for NCI and one for the Centers for Disease Control and Prevention.

The most recent publication, the Atlas of Cancer Mortality in the United States, 1950-94, uses data from earlier atlases to take advantage of all three functions. The latest effort, authored by Susan Devesa, Ph.D., from NCI’s Division of Cancer Epidemiology and Genetics and her colleagues (see News, Jan. 5, p. 15), used data from the original atlas, which covered 1950 to 1969, and compared it with mortality data from 1970 to 1994. It is also the first cancer atlas to include separate mortality maps for blacks.

To further facilitate interpretation of the maps, the online version (http://www.nci.nih.gov/atlas/) allows users to view county, state economic area, and state cancer mortality maps; view a single county, state economic area, or state and associated data; and zoom in, zoom out, and pan (see related article, p. 534).

This "dynamic" version of the cancer atlas is made possible by improvements in mapping software, statistical modeling, and hardware. Gone are the days of extracting data by hand and printing maps on plotters; today, improvements have brought mortality mapping to the desktop.

"It’s like drawing with crayons versus drawing on a computer," Pickle said.

The Future

The software that makes this possible is called geographic information systems. But even now, researchers are still working on maximizing GIS’ potential.

GIS can "layer" information; for example, one map could include information on the demographics of a region, its cancer rates, levels of exposure to carcinogens, and the lifestyle factors of its residents.

This type of information would be a tremendous resource to local health departments, said Gerard Rushton, Ph.D., professor of geography and of public health at the University of Iowa, Iowa City.

"Within its data structures, you have the information bases for all these confounders such as age, socioeconomics, etc.," he said. "GIS is then used to model the relationships that have been established at some macro level. GIS can take these macro relationships and show what they mean in local areas."

And pursuing projects like these is a priority for NCI. Later this summer, NCI will announce its intent to fund studies that follow up on the latest cancer atlas and that look at GIS applications in research.

Cancer mapping will also move beyond cancer mortality. As tumor registries improve, incidence maps will be possible. And there’s no need to stop there, Rushton said. "One can map things like stage at diagnosis. How does that vary geographically? How well is the health system performing?"

GIS software can also take cancer maps to three dimensions, allowing a "virtual" tour of a region’s rates, exposures, and lifestyle factors—a far cry from Snow’s plots of cholera cases and Mason’s hand-drawn mortality maps.

"Some people look at this and think its just a gee-whiz thing," said Pickle. "The virtual tour would allow to zoom into a very localized area to focus research. We need to improve the data and statistical methods so we can utilize these new tools in cancer research."



             
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