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.
