Monday, July 29, 2013

Should you be screened for lung cancer? Maybe not, and here's why

If you are a lifelong heavy smoker age 55 years or older, the U.S. Preventive Services Task Force believes that screening for lung cancer with CT scans may save your life. Today the Task Force released provisional recommendations that assigned a "B" letter grade to this preventive service, which, if eventually finalized, would place CT scans alongside established cancer screening tests such as mammograms and Pap smears and mandate that health insurers cover these scans without cost sharing for eligible patients.

The scientific basis of the USPSTF's new recommendations is a large U.S. study sponsored by the National Cancer Institute that concluded in 2011 that heavy smokers randomly assigned to screening with CT scans had a 20 percent lower relative risk of death than heavy smokers randomly assigned to screening with chest x-rays. However, a more recent analysis by the same researchers found that the absolute benefit of CT screening in this study varied greatly according to a patient's baseline risk of lung cancer death. For the highest-risk patients, the chance of CT scans preventing death from lung cancer is about 1 in 100 (a 5% risk of lung cancer death over 6 years without screening minus a 4% risk with screening); for lower-risk patients, that chance is about 1 in 5000 (a 0.1% risk of lung cancer death without screening minus a 0.08% risk of death with screening).

For patients in the lower-risk group, for whom the potential lifesaving benefits of CT scans are very small, the downsides of the screening test become considerably more important. Screening tests have harms just like any other medical procedure, and it's important for your doctor to thoroughly review those harms with you if you are considering screening. In my mind, there are at least 4 good reasons for current or former smokers to think twice about being screened for lung cancer:

1. The risk of developing cancer from the CT scan itself isn't trivial. An analysis published in the Archives of Internal Medicine found that a typical chest CT scan exposes patients to the radiation equivalent of more than 100 chest X-rays, and that at age 60, an estimated 1 in 1000 women or 1 in 2000 men would eventually develop cancer from that single scan. Although some imaging centers now use lower radiation doses, repeating these lower-dose CT scans annually still adds up. (It hasn't been long enough since the conclusion of the NCI's lung cancer screening study to measure how much these scans increased the participants' risk for other cancers.)

2. False alarms are extremely common. In the NCI's study, more than 96 percent of all positive results turned out to be false positives, and in a previous CT screening study, 1 in 3 patients had at least one false-positive result after undergoing only two CT scans. Of those patients, 1 in 14 needed an invasive lung biopsy to be sure they were cancer-free. Such diagnostic procedures for lung cancer can themselves be life-threatening: in 2005, former Canadian prime minister Brian Mulroney (a longtime heavy smoker) spent several weeks in the intensive care unit after postoperative complications from surgery to remove two lung nodules found on a screening CT scan that turned out to be non-cancerous.

3. A CT scan for lung cancer could find some other unrelated abnormality that will require further investigation; in the NCI's study, this occurred in about 1 in every 13 patients. You might think this is a good thing, but most of these abnormalities (known as "incidentalomas") turn out to be false alarms, too. In fact, in 2008 this very same Task Force decided against endorsing CT screening for colorectal cancer due to concerns that invasive testing to definitively diagnose all of the abnormalities that CT scans turn up could easily outweigh the cancer-prevention benefits.

4. Finally, even if screening catches a true lung cancer early, there's no guarantee your prognosis will be better. This is due to "overdiagnosis," or the unnecessary diagnosis of a condition (typically cancer) that will never cause symptoms in a patient's lifetime, either because it's so slow-growing or the patient dies from some other cause. (Statistics show that most lifelong smokers will die from heart disease, not lung cancer.) An estimated 1 in 3 breast cancers detected by screening mammograms is overdiagnosed, and a 2007 study published in the journal Radiology suggested that the proportion of lung cancers overdiagnosed by CT scans could be as high or higher, especially in women. But because there's no way of knowing at the time of diagnosis if a lung cancer will be fatal, inevitably most of these patients will be needlessly subjected to the side effects of treatment - making the "cure" worse than the disease.

I expect that some of my patients will decide to be screened with CT scans for the obvious upside emphasized by the U.S. Preventive Services Task Force: a small chance of preventing death from lung cancer. But before they make this decision, I will counsel them to carefully consider the more likely downsides, and only choose testing if they're prepared for these too.


This post's title is modeled after the title of the excellent book Should I Be Tested for Cancer? Maybe Not and Here's Why by H. Gilbert Welch, MD, MPH. Portions of this post are revised and updated from a previous post on Common Sense Family Doctor.