A new study from the National Cancer Institute (NCI), part of the National Institutes of Health, shows that screening for lung cancer with chest X-rays can detect early lung cancer but also can produce many false-positive test results, causing needless extra tests. This report, which summarizes preliminary results from the Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial, appears in the December 21, 2005, Journal of the National Cancer Institute.
About 170,000 people in the United States are diagnosed with lung cancer each year. Most are diagnosed when their disease is advanced, and nearly 90 percent die within two years. But catching lung cancer early — when surgery is a treatment option — improves survival substantially, and 70 percent of patients who are diagnosed early may survive at least five years.
Between 1993 and 2001, PLCO investigators enrolled 154,942 men and women who were 55 to 74 years of age. These participants included current and former smokers, as well as individuals who never smoked. These findings, the first published lung cancer screening results from the PLCO, are based on an analysis of the trial participants’ initial chest X-rays. It is the first large, controlled study to evaluate screening for lung cancer in women, whose smoking rates have increased in recent years.
“There is no accepted early screening technique for lung cancer,” said Christine Berg, M.D., the NCI investigator who leads the PLCO trial. “The PLCO trial will show if chest X-rays, by catching lung cancer when it is still operable, can reduce the death rate from lung cancer.”
Of the 67,038 men and women who received a baseline chest X-ray upon entering the trial, 5,991 (8.9 percent) had abnormal results that required follow-up. After undergoing additional tests, 126 (2.1 percent of the 5,991 participants with abnormal X-rays) were diagnosed with lung cancer within 12 months of the initial chest X-ray.
“The positive predictive value was low,” said Berg. “That means there were a lot of false positives on the initial X-rays. If you get a positive result from a chest X-ray, the message is ‘don’t panic.’” Berg also noted that tissue variations and other benign factors can resemble tumors on an X-ray.
Of the cancers detected, though, 44 percent were stage I, meaning those patients were good candidates for surgery. “The rate of early cancer detection was better than what we see in the general community,” said Berg. “But it remains to be seen if that translates into a mortality benefit. It is too early to make any recommendations regarding chest X-rays as a lung cancer screening tool in the general population.”
Chest X-rays have not been shown to reduce the death rate from lung cancer, according to several previous studies. Because of their smaller enrollments, however, these earlier studies may have overlooked a small but important benefit of annual chest X-rays. One of the long-term goals of the PLCO trial is to determine whether chest X-rays can reduce lung cancer mortality in men and women 55 to 74 years of age.
Participants in the intervention group of this study — those who received an initial (baseline) chest X-ray to screen for lung cancer — are being tracked alongside participants in the control group — a group of equal size that did not receive screening chest X-rays. Future analysis will reveal if the intervention group has a lower lung cancer mortality rate than the control group.
The current analysis confirmed that smoking vastly increases the risk for lung cancer. Of current smokers in the trial, 6.3 people per 1,000 screened were diagnosed with lung cancer. For former smokers (those who quit for less than 15 years), the lung cancer detection rate was 4.9 per 1,000. Among non-smokers, lung cancer was diagnosed in 0.4 individuals per 1,000 screened. Lung cancer rates were similar for men and women smokers, but, for unknown reasons, men had more positive X-rays (9.6 percent) than women (8.2 percent).
“The take-home message is to never start smoking, and if you do smoke, to quit,” said Berg.
Another NCI-supported study, the National Lung Screening Trial (NLST), is comparing two ways of detecting lung cancer: spiral computed tomography (CT) and standard chest X-ray. Both chest X-rays and spiral CT scans have been used to find lung cancer early. So far, neither chest X-rays nor spiral CT scans have been shown to reduce a person’s chance of dying from lung cancer. This study will aim to show if either test is better at reducing deaths from this disease.