Understanding the Drop in Cancer Cases

Study co-author Dr. Ahmedin Jemal, an epidemiologist with the American Cancer Society, said he expects the number of women who develop lung cancer to drop in the years to come as more women, like Gonzales, choose to kick the smoking habit. But he said more support in the form of anti-tobacco legislation and state spending on smoking prevention efforts is needed to spur progress.

"The decrease in tobacco use has provided a significant opportunity to prevent cancer," he said. "Smoking accounts for nearly one-third of cancer cases, and yet many states are not properly funding tobacco control programs."

Specifically, Jemal noted, states will likely spend less than 3 percent of the $25 billion in funds they will get from the Tobacco Master Settlement Agreement of 1998 on anti-smoking efforts, a deal originally between the four largest U.S. tobacco companies and the attorneys general of 46 states. Overall, 43 million Americans continue to smoke, leaving much room for reduction in the cancers among this segment of the population.

For cancers not linked to smoking, the reasons for the downturn become slightly murkier. The decreases in colorectal cancer seen in both men and women are most likely due to improved detection, said. Dr. Michael Cummings, chairman of the Department of Health Behavior at Roswell Park Cancer Institute in Buffalo, N.Y.

"For colorectal cancer, my guess would be the declining incidence is the result of screening where polyps are found and removed ... thereby preventing cancer," Cummings said. "This essentially is an example of surgical prevention, which is very effective."

Dr. Leonard Zwelling, professor of medicine and pharmacology at the University of Texas M.D. Anderson Cancer Center in Houston, said, "Assuming this is a true drop ... people could be doing healthier things with diet and exercise; screening may be picking up early precancerous lesions and preventing them from becoming cancer by excision -- particularly colorectal."

As for prostate cancer, doctors said we could now be reaping the benefits of preventive procedures performed in the past decade.

"We 'harvested' a huge number of existing prostate cancers with PSA screening in the 1990s," said Dr. Tim Byers, deputy director of the University of Colorado Cancer Center in Aurora, Colo. "Thus, we are seeing reduced incidence now, compared to that prior period of inflated incidence."

And cancer experts speculated that a number of factors may be behind the reduction in breast cancer incidence.

"For breast cancer, most likely this relates to something that influences estrogen levels," Roswell Park's Cummings said, adding that "changes in the use of hormone replacement therapies and birth control pills" since the beginning of this decade could be part of the answer.

But when it comes to less prevalent cancers, such as liver cancer, rates are still on the rise. Study co-author Jemal said that for liver cancer, the underlying cause could be rising obesity rates. Alternatively, higher rates of infection with the hepatitis virus could be a factor.

As for kidney, thyroid and other cancers, Jemal said, "I don't really know why there is an increase."

Even the decrease seen in certain cancers may not be entirely good news, Texas' Zwelling warned.

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