NEWS

Statistics Offer Insights Into Progress Against Cancer

Katherine Arnold

    "... It is the sense of the Congress that the conquest of cancer is a national crusade to be accomplished by 1976 ... ."

    —House Concurrent Resolution 526, March 4, 1970

In the early 1970s, many supporters of the National Cancer Act believed that the intense efforts outlined in the legislation could lead to a quick and finite end to the war on cancer. The provisions of the act proved fruitful, but in an unexpected way: Within a decade, researchers learned that cancer was more complex than had ever been imagined, and an end to that war was nowhere in sight.

Exactly how much progress has been made in that "conquest" depends on one’s perspective. The National Cancer Act provided the National Cancer Institute with a budget of $400 million in 1972; this year’s budget was $4.6 billion. Three decades ago, chemotherapy, radiation therapy, and surgery were a patient’s only options for treatment; today, targeted drugs are attacking cancer at specific points in the malignancy process. About 9.6 million people with a history of cancer are alive today; this is up from 8.2 million just 5 years ago.

These facts may shed some light on the status of cancer research as a whole, but the bottom line is that people are still getting cancer and are still dying of the disease. Therefore, the gold standard for charting progress against cancer is an analysis of population statistics about cancer, such as those reported in this issue of the Journal in this year’s Report to the Nation (see article, p. 1276). But even with hard numbers and 25 years of trend data, the picture is not crystal clear.

Mortality

Few will argue with the notion that mortality is the most telling and authoritative statistic in measuring progress against cancer. It reveals patterns of the risk of dying of cancer, said Brenda K. Edwards, Ph.D., associate director of the National Cancer Institute’s Surveillance Research Program. "The mortality rate reflects what has happened to our population in terms of individuals who were diagnosed many years ago," Edwards said. "Because those who may be dying today of cancer reflect what happens to them at the time they were diagnosed and the treatments that were available, it’s a mixture of our history with diagnosing and treating cancer."



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Dr. Brenda Edwards

 
Mortality is also an easy concept to understand, and it is one that the public is most concerned about, noted John C. Bailar III, M.D., Ph.D., who has published several articles in the last two decades examining cancer mortality rates as an indicator of progress against cancer.



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Dr. John Bailar III

 
Another strength is that death is very precisely recorded. "It’s not fuzzy like trying to measure symptoms or quality of life," said Bailar, professor emeritus in the Department of Health Studies at the University of Chicago. "Death certification is now very good throughout the whole country, so we have a really precise handle on what’s going on."

But recent changes in how deaths are recorded may actually have contributed to a shift in the mortality trend in the last few years. Cancer mortality increased steadily throughout the 1970s and 1980s and then decreased from 1994 to 1998, before stabilizing from 1998 to 2000. The authors of this year’s report noted that changes in the classification system for recording deaths—the adoption of the ICD-10 codes—was responsible for an estimated 0.7% increase in the number of deaths attributed to cancer in 1999 and 2000. This may have had an artifactual effect on the actual cancer rates.

So mortality rates are not completely immune to influence from outside factors. In addition, although mortality is the best indicator for overall progress, it is also a late indicator, noted Peter Greenwald, M.D., director of NCI’s Division of Cancer Prevention. Worthy advances in prevention or treatment eventually have an effect on the mortality rate, but the effect may be delayed by years.



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Overall cancer incidence and mortality rates began to stabilize in the mid- to late 1990s. The mortality rate is the most telling and authoritative statistic in measuring progress against cancer.

 
And the statistic gives no indication of other areas that affect all cancer patients. "Death cannot record some of the important effects of cancer—the progress of disease, the disability, the symptoms that can be very distressing—nor can it directly measure the good and bad effects of treatment," Bailar said.

Incidence

NCI’s Surveillance, Epidemiology, and End Results (SEER) program has been collecting information on cancer incidence, or the rate of new cases of cancer diagnosed each year within the population, for 30 years. Through state registries, the program has tracked 10% of the cancer cases in the United States since 1973 and 26% since 2001, with several expansions in between. These data are then extrapolated to the entire population. Overall cancer incidence increased from 1975 to 1992, declined from 1992 to 1995, and has been stable ever since.

As a predictor of progress in cancer prevention, incidence is the most solid measure, said Greenwald. "If you lower the incidence rate, you’re stopping occurrence in cancer in the first place," he said. For example, changes in smoking patterns have a very predictable impact on lung cancer incidence, although the effects are not seen until many years later.

Screening tests for cancers with known precursors, such as cervical cancer and colorectal cancer, have had or will have a measurable effect on cancer incidence, he added. "When you screen, you find precancerous lesions and you remove them. ... In a sense, removing those [precancers] is prevention because you’re preventing invasive cancer." The ultimate effect, it is hoped, is a decline in incidence of those diseases.

But for many cancers, there are no ways to detect precancers; instead, new or better detection methods find actual cancers and thus increase the incidence rate. Such was the case for breast cancer rates and prostate cancer rates in the 1990s; increased availability of screening tests increased the number of people who were diagnosed with the disease.

"By looking at our incidence data or trends, we have more insight into the mix of cancer types that are currently being diagnosed and treated," Edwards said. "We also get a sense about the way in which we may be seeing shifts in the extent of disease."

Survival

Survival, or the measure of the number of people who have or have had cancer who are still alive after a certain time period, is on its surface easy to understand. For example, in 2000, England’s National Health Service summarized survival improvements in its Cancer Plan: "Overall, the number of people surviving more than 5 years has improved—an average of 4% every 5 years," the report stated. "These improvements are almost certainly due to a combination of earlier diagnosis and better treatment."

But those reasons are precisely why survival is not a meaningful statistic for comparisons over time and between nations, say Steve Woloshin, M.D., and Lisa Schwartz, M.D. In 2000, Woloshin, Schwartz, and colleague Gilbert Welch, M.D., all from the VA Outcomes Group at the Department of Veterans Affairs Medical Center in White River Junction, Vt., looked at the relationship between cancer mortality and 5-year survival rates in 20 cancers. They found that the 5-year survival rates had increased for all 20 cancers, but the changes in mortality varied—some increased, some decreased, and some remained unchanged.

"Changes in the 5-year survival rate are not a proxy for changes in mortality," Schwartz said. "The only way to really know if there’s improvement is to look at changes in death rate."

For example, 5-year survival for prostate cancer in whites improved from 43% in 1950 to 98.4% in 1999. Over the same time period, mortality declined a mere 4%. On the other hand, 5-year survival for stomach cancer increased from 12% in 1950 to just 21% in 1999, and mortality rates plummeted 83% during those years.

Survival data can be particularly useful in the clinical trials setting when comparing cancer therapies, Schwartz said. "Survival data is great for looking in a randomized trial where everybody starts out at the same stage of disease," Schwartz said.

But problems arise when comparing survival rates of people who may have been diagnosed at different stages of their cancer. "In the 1950s, people with prostate cancer were diagnosed because they had symptoms, and now they’re diagnosed because of a PSA test before they are symptomatic. So the comparison [between survival rates across time] is inherently biased," Woloshin said.

The Overall Picture

The SEER program has been monitoring cancer rates for 30 years and continues to revise and update its procedures for estimating cancer rates to better reflect changes in the population. Even with changes in cancer mortality and incidence, the actual count of people dying from and being diagnosed with cancer continues to escalate because the population of the country is aging. "The rate is going down, but the count is going up," Edwards said. "How can these things co-exist? They can, just because of the nature of the population phenomenon."

For mortality and especially for incidence, overall rates present a simplistic interpretation of a very complex picture. For example, there has been a 1.8% per year decrease in lung cancer incidence among men. Because lung cancer rates represent such a large percentage of the overall cancer burden, even small fluctuations in lung cancer rates can affect cancer rates overall. However, according to this year’s Report to the Nation, the decrease in lung cancer rates has been offset by a 0.6% per year increase in breast cancer incidence and a 2.3% per year increase in prostate cancer incidence.

"Each of these measures is related to the other, but probably in a more complicated way than people understand," Edwards said.



             
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