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How Questions About Mammography Apply To CT Scans For Lung Cancer

Army Captain William Krueger studies CT scans at the National Naval Medical Center in Bethesda, Md. CWO4 SETH ROSSMAN/NAVY VISUAL NEWS SERVICE hide caption

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Here we go again.

With the latest round of the mammography wars still fresh in mind, there's solid evidence that a different technology can take a slice out of lung cancer mortality — a 20 percent reduction among people at highest risk.

But take a deep breath and look a little more closely at the implications of the big new federal study on CT lung screening.

For better or worse, CT lung screening and mammography are going to have a lot in common.

Strikingly, the 20 percent mortality reduction found for lung CT screening is in the same ballpark as the probable benefit from mammograms – which reduce breast cancer deaths by 10 to 30 percent, depending on the age of those screened. So if mammography has set a cancer screening standard, lung CT passes that test.

But lung CT also shares strikingly similar downsides with mammography.

Consider this: The study found that one life is saved from lung cancer for every 300 people screened. As Dr. Peter Bach at Memorial Sloan-Kettering Cancer Center notes, this is an absolute mortality risk reduction of only one-third of one percent.

That benefit comes at a real clinical cost, though. Twenty-five percent of those 300 people — 75 in all — will get a positive CT screen that most likely won't be  cancer. That's called a false positive result.

The false positive rate for screening mammograms is the same — 24 percent of women will have at least one false positive mammogram over a 10-year period, according to one study.

Just as with mammography, people who get a suspicious result with lung CT are in for an emotional roller-coaster ride. They'll be asked to come back for another CT three to six months later to see if the suspicious lump has grown. If it has, or if the next test is equivocal, they might need a needle biopsy — a long needle stuck into the lung to retrieve a bit of the suspect tissue. And some will need open-chest exploratory surgery.

One recent study found that the false-positive rate for lung CT is 33 percent among those who have had two screening tests – higher than the new federal study found. And among these, seven percent will end up having an invasive followup test — a biopsy or surgery.

Another implication: The cost of each lung CT (several hundred dollars according to the National Cancer Institute) is only the tip of the iceberg, just as with mammography. For every $100 spent on mammography screening, add another $33 to follow up on false positives. It's not clear how much the equivalent add-on will be for CT lung screening.

Stepping back, think about how this applies to the staggering 92 million current and former smokers in this country who might be candidates for lung CT screening.

The results from lung CT study applies only to the heaviest smokers and former smokers among them — but that's a high proportion. And if lower-risk people seek lung scans, their chances of reducing their risk of lung cancer death will be lower. But their risk of false positives and all they entail will be the same, or perhaps higher.

None of this is to say we shouldn't get the most benefit possible out of lung CT screening – with the least harm. But right now, it's just not obvious how to accomplish that.