What I am not saying
1. I'm not saying that rising five-year survival rates are never good news. Just because cancer testing which leads to earlier diagnosis and the increased likelihood of finding nonprogressive cancers will always increase five-year survival, that doesn't mean that rising five-year survival is always only an artifact of cancer testing. Increased five-year survival may reflect 1 or 2 pieces of "good news." First, new cancer treatments may be genuinely more effective than old ones. Second, old treatments may simply be more effective when we begin them earlier (thanks to cancer testing).
But for either of these pieces of "good news" to be true, cancer mortality should drop. We have seen this happen, most dramatically in testicular cancer, and we may be realizing it in breast and colon cancer. The bottom line is that meaningful improvements in detection or treatment ought to affect not just five-year survival but also mortality. 2. I'm not saying that five-year survival is always an ambiguous statistic. In a randomized trial of two cancer treatments, the treatment producing the longer five-year survival is without doubt more effective. Here's how a randomized trial of therapy works: Newly diagnosed cancer patients are invited to join the study.
Those who agree are randomly assigned one of two treatments. The five-year survival clock starts at the same time in both groups: at the time of enrollment. Because the two groups are essentially the same except for treatment received and because the clock starts at the same time, we can be confident that the group with the higher five-year survival rate did in fact put off death. So five-year survival is a perfectly valid measure in a randomized trial of cancer (or any other) therapy.
Cancer testing has a powerful effect on five-year survival rates. Individuals diagnosed with different diagnostic tests and at different times in the course of their disease should always be expected to have different five year survival rates. Expect higher five-year survival rates in patients diagnosed with new, more sensitive tests and lower ones in patients diagnosed with old tests. Expect higher five-year survival rates in patients with early-stage cancer and lower ones in patients with late-stage cancer. And because every year we find cancers earlier, expect that five-year survival rates will continue to increase over time, for all cancers.
None of this, however, tells you anything about the usefulness of cancer testing (or the usefulness of treatment). For that you have to look elsewhere. Surprisingly, a fair number of doctors don't understand the problems associated with five-year survival rates. And I am sorry to report that a number of them do research, research that gets published in the medical literature and even reported in the general press. Here's an example, from the Toronto Star:
The Boston study compared the outcomes for 117 women age 40 to 49 whose breast cancer was diagnosed by mammography with that of 928 women whose cancer was detected by physical examination. It found that 40 per cent of the women whose cancers were diagnosed by mammography had precancerous tumors that had not spread into surrounding tissue while only 9 per cent of women in the physician exam group had this very early type of tumor. The five-year survival rate for women whose tumor was found by mammography was 96 per cent compared with 74 per cent for patients in the other group.
You've now learned what this does and doesn't tell you. It tells you that mammography finds smaller cancers than a physical exam. You expect that. It tells you that women whose cancers were diagnosed by mammography were more likely to be alive five years after diagnosis than women whose cancer was diagnosed by physical exam. You expect that too. It doesn't tell you whether finding cancer early delayed anyone's death. It also doesn't tell you something else that is essential: Were all the cancers diagnosed by mammography destined to become a problem?
I can pretty much guarantee that you'll see the same basic argument over and over maybe involving a different test and a different cancer, but the pattern is predictable. Step 1 is to use a cancer test to find earlystage cancers. Step 2 is to observe that the test does what it is supposed to do: find patients whose cancers are at earlier stages than patients whose cancer was found without the test. Step 3 is to observe that patients whose cancer was found by the test have a higher five-year survival rate. Step 4 is to call for screening using the test.
This pattern has recently appeared in the medical literature, with UCLA researchers calling for kidney cancer screening and Cornell researchers calling for lung cancer screening
. The Cornell findings quickly made it to the New York Times, where among the researchers was quoted as saying, "The method [spiral CAT scans[]|] could allow as many as 80% of lung cancer patients to survive at least five years. Just 15% live that long now." Spiral CAT scan technology turns up about 10 times as many lung cancer cases as do chest X-rays, and finds almost as many lung cancers in nonsmokers as in smokers. Quite simply, it finds lots of lung cancer we never knew existed.
I'm afraid that the bottom line is this: When you hear the term five-year survival, you need to be suspicious. If it appears in the context of a randomized trial of cancer therapy, you may be confident that the measure is valid. But if the term is juxtaposed with a cancer test or the words early diagnosis or screening, alarms should go off. Either someone is trying to persuade you with a comparison they know is faulty or they really don't know what they are talking about.
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08152010
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