Thursday, November 2, 2006

From the Archives (11-2-06): Don’t Jump the Gun: Lung Cancer Screening with Spiral CT


A recent study in the New England Journal of Medicine on lung cancer screening certainly made a big splash in the news last week.  Whether it actually deserves such attention, though, won’t be known for a while. 

The large study of over 31,000 people at increased risk of lung cancer looked at rates of survival in those found to have earliest stage lung cancer (stage I) using a screening tool called spiral computed tomography, or CT.  Spiral CT provides a more detailed image than chest x-rays so may possibly catch lung cancers in early stages when they may be more curable.

What the researchers found was that spiral CT did in fact find cancers early—85 percent of the cancers found were stage I compared to the national average of only 15-20 percent—and that those subjects with stage I cancer had very good projected survival rates—88 percent for 10 years compared to the national average of 70 percent for 5 years. 

On its face, this seems like an amazing result and a ringing endorsement for widespread lung cancer screening with spiral CT, but this study actually has some important drawbacks. 

First, the reported survival rates are projected estimates, not actual observed survival rates.  This means that they’re extrapolated from a shorter amount of real time follow-up.  So even though the paper reports estimated 10 year survival, the researchers actually only followed cases for an average of 3 years and 4 months.  Although such projections can sometimes be accurate, they can also be wrong, and they’ll never be as exact as following people for the full ten years.

Second, the study did not have a comparison group of people who were not screened using spiral CT.  This means there’s no way to actually know if those who have spiral CT screening actually live longer than those who don’t.  It could be that many of the small tumors spiral CT finds would grow very slowly and never develop into life-threatening tumors within a person’s lifetime.  Or, it could be the opposite, that some serious tumors found early on spiral CT would be just as incurable no matter when or how they were found .

In addition, there are also unanswered questions about the risks of using spiral CT to detect lung cancer in the larger community.  Because spiral CT provides such a detailed image of the lung, it can have a high false positive rate—meaning it can find a lot suspicious masses that turn out not to be cancer. To rule out cancer, though, these masses often have to be followed up with other, sometimes risky tests, like a lung biopsy. Whether such risks are worth the benefits of spiral CT is unclear.

These are the types of issues that can only be resolved by long term randomized  clinical trials that compare spiral CT screening to either standard chest x-rays or no screening at all.  And a number of these trials are currently running in the US and Europe. Only when these studies confirm the overall benefit of spiral CT will we know if it can really live up to its recent headlines as a life saver.

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