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5-Year Survival Data Not Always a Good Measure of Progress
The 5-year survival rate used to measure progress in the current SEER analysis of adolescent and young adult cancer prognosis is controversial. Although the statistic is common in cancer research, many experts suggest that it can also be prone to biases and misleading results.
Of chief concern is that 5-year survival rates are heavily influenced by the time of diagnosis, says H. Gilbert Welch, M.D., who codirects the Veterans Administration Outcomes Group at Dartmouth Medical School in New Hampshire. To illustrate how that skews their interpretation, he offers the following example: Consider two individuals who both contract cancer at age 42 and then die from it at the age of 50. However, one isn't diagnosed until age 47, while the other is diagnosed at age 42. The first patient lives 3 years with a cancer diagnosis and contributes nothing to 5-year survival, while the second patient lives with the diagnosis for 8 years. Nevertheless, both patients live with cancer for the same duration and die from it at the same age.
"The data would lead you to think the second patient had a big improvement, but the reality is that the patient just lived with a diagnosis for a longer period," Welch explains.
Another problem, he says, is that some cancers meet pathological definitions for the disease and yet progress slowly or not at all. "And those cases inflate the statistic and distort its meaning," Welch says.
The NCI acknowledged bias issues with the 5-year survival rate back in 1990. In a report titled Special Report: Measurement of Progress Against Cancer, published by JNCI (1990;82:825–35), a panel of scientists from NCI and academia suggested that survival rates have biases that " ... may result in apparent trends in survival over time that do not reflect meaningful changes to the population." Echoing Welch's view, that same panel wrote, "Mortality rates remain the most important measure of progress against cancer."
Crystal Mackall, who currently heads the immunology section at NCI's pediatric oncology branch, emphasizes that 5-year survival rates have limitations that vary by disease. For rapidly progressing cancers, she says, the measure has more value than it does for indolent cancers that progress slowly.
Nonetheless, Mackall stresses that despite their reliance on 5-year survival data, the SEER conclusions on adolescent and young adult cancer prognoses are compelling. "The data aren't perfect, but to me it looks like their outcomes are substantially reduced," she says. "I think this is a wonderful starting point, and I'm now more convinced than ever that more work needs to be done."
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