A Screening Test for Lung Cancer
This week brings very exciting news, but everybody seems worried that we’ll misunderstand and read too much into it.
There are very few cancers for which we have a good screening test. A good screening test is a test that is done on people without any signs or symptoms of cancer and that diagnoses the cancer accurately enough at an early enough stage so that lives are saved. Mammograms save lives from breast cancer. Pap smears save lives from cervical cancer. Screening for colon cancer saves lives. And that’s about it – three cancers that we can do something about before signs or symptoms show up. Prostate cancer screening remains controversial and we hope for more definitive answers in the next few years. But for all the other cancers (lymphoma, sarcoma, esophageal cancer, testicular cancer, kidney cancer, ovarian cancer, leukemia…) there is no evidence that any test is better than diagnosing the disease after symptoms are present.
For decades, a screening test for lung cancer has been sought. Though the incidence of lung cancer has been decreasing as fewer Americans smoke, it remains the leading cause of cancer death. In the 1970s chest X rays were studied as a way to screen smokers for lung cancer. Unfortunately chest X rays were found not to save any lives. The patients found to have lung cancers by chest X ray died of their cancer as frequently as those whose cancer was diagnosed when it was symptomatic. (Tangentially, that means that if you’re getting routine annual chest X rays your doctor is only 40 years behind the medical literature. He is sure to catch up soon.)
This week a very large study called the National Lung Screening Trial showed that in the right patient population CT scans effectively screen for lung cancer. The trial enrolled 53,000 people who were
- aged 55 to 74,
- were either current smokers or had quit in the last 15 years,
- had not been diagnosed with lung cancer, and
- had smoked at least 30 pack-years.
What’s a pack-year? It’s 365 packs of cigarettes, or how much you smoke by smoking a pack per day for a year. So 30 pack-years is how much you smoke by smoking 2 packs per day for 15 years, or 3 packs per day for 10 years. The point is that risk from smoking is cumulative and is proportional to the total lifetime amount of cigarettes smoked.
The trial randomized the 53,000 people into two groups. One group received annual chest X rays; the other received annual helical (also called spiral) CT scans of the chest. Both groups were followed for five years. In the group randomized to CT scans 354 died of lung cancer, compared to 442 in the chest X ray group. This was a large enough difference that the trial was stopped early, which means that the actual benefit of CT scans might be even larger than these numbers suggest.
This means that for every 300 people screened with CT scans one life was saved from lung cancer. That’s pretty good. And it’s now the fourth cancer for which (in the right patient population) we have a reasonable screening test.
Unable to leave good news alone, all the national cancer gurus are terribly worried that two misunderstandings will arise from these results. So let’s clear them up right now.
Does this mean it’s safe to smoke now? No! First of all, CT scanning only managed to decrease mortality from lung cancer by 20%. That’s not even a quarter. Decreasing the mortality of running across the freeway by 20% doesn’t make it safe. That rest of the mortality – that big 80% – is still there. Secondly, this doesn’t eliminate all the other risks of smoking: emphysema, heart disease and stroke.
Does this mean that everyone should get a spiral CT? No! Take a second look at the criteria listed above of the patients enrolled in the study. That is the only group of people for whom we know screening is effective. There is no evidence that CT scanning will help anyone outside this group.
The actual scientific paper has not yet been published. So there are a lot of details and numbers that doctors are eagerly awaiting before we can answer specific questions. But for now it looks like there’s a fourth cancer that we can effectively catch early.
(Thanks to Robert C. and Timo K. for pointing me to his story.)