Writing in the New York Times on Sunday, Paul Kalanithi—a Stanford University neurosurgeon who was recently diagnosed with lung cancer—examined why doctors often refuse to answer the common patient query, "How long have I got left?"

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After having many discussions with patients and their families about grim prognoses, Kalanithi writes that he's learned a few basic rules:

• "Be honest about the prognosis but always leave some room for hope."
• "Be vague but accurate: 'days to a few weeks,' 'weeks to a few months.'" Never cite detailed statistics.
• Counsel patients against Googling survival numbers, lest they misunderstand the data.

Kalanithi discourages patients from looking up five-year survival rates—called Kaplan-Meier curves—because they are "at least five years out of date" and lack necessary context. Though such curves indicate that, although average survival numbers haven't changed significantly, a few patients are living longer, "The problem is that you can't tell an individual patient where she is on the curve," he writes.

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Kalanithi's take on survival statistics shifted dramatically when he crossed the line from doctor to patient.  

After being diagnosed with lung cancer, "I had the same yearning for the numbers all patients ask for," he recalls, adding that his specialist "always avoided being pinned down to any sort of number." Kalanithi began to wonder "why physicians obfuscate when they have so much knowledge and experience." He grappled with large, general studies that found between 70% and 80% of lung cancer patients die within two years.

Given his diagnosis, poor health, and test results, he though that "[t]he next steps were clear: Prepare to die." But then, his health improved, and his future became uncertain. "In a way, though, the certainty of death was easier than this uncertain life," Kalanithi writes.

"I began to realize that coming face to face with my own mortality… had changed both nothing and everything," Kalanithi writes. Both before and after the cancer diagnosis, he was aware that "someday I would die, but I didn't know when."

Doctors cannot give patients specific prognoses because "[f]aced with mortality, scientific knowledge can provide only an ounce of certainty," Kalanithi says. It's nearly impossible to predict how long a patient will live based on today's therapies, and it's even more difficult to measure based on new treatments that will be available within two to three years.

But more importantly, "What patients seek is not scientific knowledge doctors hide, but existential authenticity each must find on her own." Kalanithi compares patients' desire for hard statistics to "trying to quench a thirst with salty water. The angst of facing mortality has no remedy in probability" (Kalanithi, New York Times, 1/24).