In June, the U.S. government reported declines in overall cancer incidence rates for the fourth year in a row. Despite somewhat similar demographics in western European populations, cancer rates there have not followed suit, and instead show an overall increase.
Lynn Ries, Ph.D., of the National Cancer Institutes Surveillance Research Program, attributes the overall decline to prostate cancer screening. The United States started using prostate-specific antigen to screen for prostate cancer in the late 1980s. This drove up incidence rates that now are coming back down, she said.
D. Maxwell Parkin, M.D., of the International Agency for Research on Cancer, Lyon, France, concurs with Ries analysis. "PSA testing picks up latent disease in people who would otherwise be unaware of it, leading to more positive tests and higher incidence."
He said that in Europe there has been no big movement to test older men for PSA levels, although the rates of PSA testing throughout Europe are somewhat variable.
Lung cancer comparisons between continents can also bias analyses. For example, smoking rates in Spain and Portugal for women are still quite low because the older generation of women in those countries did not smoke and the younger generation is only just now beginning to smoke. In Britain, lung cancer rates are starting to come down for men just like the rates seen in the United States for men.
Overall, western European lung cancer rates "are going up fairly quickly and nearing a peak, hopefully, but havent started to decline like American rates have," said Parkin.
Standard Populations
Ries said she is not comfortable making exact worldwide comparisons because "it is difficult to compare rates without a world standard adjustment." To make meaningful comparisons among countries, rates are age-adjusted to the world standard population. Age-adjustment eliminates differences in rates when the population of one country has a different age distribution from that of another country. This kind of adjustment is often hard to make and sometimes difficult to calculate, but is necessary to do because "we have to compare equivalent age structures or else well get skewed analyses," said Parkin.
Parkin recently published a global cancer statistics review for the year 2000 in Lancet Oncology that incorporated these kinds of adjustments. "For Nordic countries, there are high-quality incidence and mortality statistics available nationally, whereas for several less developed countries theres much poorer or even a complete lack of data," said Parkin.
Parkin noted that "life expectancy plays a large role in how cancer rates are evolving and how they may affect comparisons." The proportion of elderly people in 2000, which was about 14% worldwide, is expected to increase to more than 25% in 2050. In developed countries, 46% of cancers occurred in people over age 64 in 2000, and that number is estimated to hit 71% (based on population projection models) in 2050."
The National Cancer Institute of Canada recently did a comparison of international cancer statistics for 1998 under the chairmanship of John R. McLaughlin, Ph.D., of the Samuel Lunenfeld Research Institute at Mount Sinai Hospital, Toronto.
McLaughlins Canadian comparison shows closely similar incidence rates for various cancers among Canada, the United States, England, Wales, and Denmark, along with a number of other Western European countries.
For prostate cancer however, strikingly different incidence rates were reported. McLaughlin attributes these differences to four possible factors: a real difference; completeness of reporting; prevailing clinical differences in considering diagnosis; and as Parkin and Ries noted, recent changes in the practice of screening for the disease.
McLaughlin noted that Canada is one of the few countries in the world that has a system of population-based cancer registries that cover most of the countrys population, whereas other nations registries may cover only urban areas and not rural, or have voluntary instead of mandatory registration. Additionally, many cases of cancer can be inaccurately diagnosed or not even diagnosed at all in some countries, leading to data that may be less accurate than those for countries like Canada.
Other Comparison Problems
It is also tricky to make comparisons between countries whose populations are shifting because of increasing rates of immigration. Walter C. Willett, M.D., Harvard Medical School, Boston, said that international studies of migrants repeatedly confirm that people tend to adopt the cancer pattern of their new country within a period that varies from about a decade for colorectal cancer to a few generations for breast cancer.
Even with conservative rates derived by Parkin and various North American sources, Willett noted that incidence has the potential to be reduced by more than 50% for many cancers. But Parkin cautioned against trying to project too far in to the future. "For example, in breast cancer, we didnt accurately project the advancements made in treatment that have helped bring death rates down in both the U.S. and Europe."
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