Behind the Good News on Cancer, There’s the Same Old Bad News
The good news about America’s war on cancer keeps coming. Prostate cancer: Five-year survival of patients is now 94%, up from 43% in 1950. Lung cancer: Patients detected early using spiral CAT scanning expected to have five-year survival of 60%, up from 14%. Colon cancer: American patients now have a five-year survival of 60%, compared with 35% for British patients.
Does this mean that fewer men are dying from prostate cancer, that we should embrace lung cancer screening with spiral computerized tomography (CAT) scans and that cancer care in the United States is far superior to care in the United Kingdom? Maybe not. The problem is that each report uses what has become the statistic of choice--five-year survival--to deliver good news about the war against cancer. And although five-year survival rates are increasing for all cancers, it may not mean what you think.
Most of us assume that rising five-year survival means that cancer death rates are falling. But in a study for the Journal of the American Medical Assn., my colleagues and I found no relationship between changes in five-year survival and changes in how many Americans die from cancer, as measured by the number of cancer deaths per 100,000 population.
Consider the two cancers with the most dramatic increases in five-year survival since 1950, prostate cancer (from 43% to 94%) and melanoma, or skin cancer, (from 49% to 88%). Despite all the press about recent declines, the current death rate from prostate cancer is actually a little higher than in 1950, and the death rate from melanoma is more than twice as high. And then consider the case for one of the cancers with the worst five-year survival rates: stomach cancer. There has been only a trivial change in stomach cancer five-year survival since 1950 (from 12% to 19%), but it is America’s biggest cancer success story because people die at one-fifth the rate they did in 1950.
To understand how five-year survival could be so misleading, you have to understand the statistic. Five-year survival is the proportion of individuals with cancer who are alive five years after receiving the diagnosis. The current five-year survival for prostate cancer of 94%, for example, means that if 1,000 men were diagnosed with prostate cancer in 1995, 940 would be alive today. So how can five-year survival go up even when death rates don’t change? The best way to understand this paradox is to work through two thought experiments.
First, to keep the math easy, let’s consider one group of men--those who died of prostate cancer at age 78 (the median age of death for men with the disease). If everyone in that group had been diagnosed when they were 75, their five-year survival rate would be 0%. But if everyone in that same group had been diagnosed when they were 70, the five-year survival rate would be 100%. What has changed? Reported five-year survival has gone up, people are aware of a cancer diagnosis for a longer period of time, but everyone still died at age 78. What have we learned? Earlier diagnosis always increases the five-year survival statistic. Whether death is postponed is quite a different question.
The second thought experiment requires some familiarity with a very foreign concept: nonprogressive cancer. Most of us think of cancer as a relentlessly progressive disease that spreads throughout the body and ultimately kills. There are some forms of cancer, however, which do not progress and so will never cause symptoms or death. It turns out that these indolent forms occur in a number of common cancers. Autopsies of people who died from other causes often find undetected cancer: A third of adults have thyroid cancer, as many as 40% of women in their 40s may have localized breast cancer, and half of men in their 60s have prostate cancer. These are people who have died with cancer, not from cancer.
Now consider the effect of nonprogressive cancer on five-year survival. Imagine a group of 1,000 men diagnosed with prostate cancer in 1950. Back then, long before our present aggressive screening programs, all would have come to see their doctor because of symptoms. In other words, all would have had progressive cancer. If 400 were alive five years later, the five-year survival rate would be 400 of the 1,000 or 40%. Today, however, we find cancer not only in patients with symptoms, but also in people who have no symptoms (some of which is progressive, some of which is nonprogressive).
To keep the math easy, let’s assume that today a group of 1,000 men are diagnosed because of symptoms and have progressive prostate cancer. In addition, another 1,000 men are diagnosed with nonprogressive cancer. The 1,000 men with nonprogressive disease all will survive five years. So the new five-year survival rate will be 1,400 of 2,000 or 70%. But what has changed? Reported five-year survival has gone up, some people have been unnecessarily told they have cancer and thus can experience only the harms of therapy and the same number of people still die of prostate cancer. Five-year survival, in other words, depends on what constitutes a cancer diagnosis and may not reflect any change in how many people die.
The news about cancer is not all bad. There has been real progress. But the public and the press need to understand the limitations of five-year survival. And those of us who evaluate medical care can help by being clear about what we know. One thing we know is that five-year survival rates will increase over time, even if death rates don’t change, simply because we are telling people they have cancer earlier in their lives and because we are finding more nonprogressive cancer.
Another thing we know is that new diagnostic tests and aggressive screening programs will always look good using five-year survival, even if early diagnosis doesn’t help people live one day longer. And because we test so much more frequently than most other countries, we know that international comparisons using five-year survival rates overstate America’s ability to provide cancer care.
