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 can also produce many false-positive test results, causing needless extra tests. This report summarises preliminary results from the Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial.

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 per cent die within two years. But catching lung cancer early when surgery is a treatment option improves survival substantially, and 70 per cent 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, claims a NIH release.

"There is no accepted early screening technique for lung cancer. 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," said Christine Berg, the NCI investigator who leads the PLCO trial.

Of the 67,038 men and women who received a baseline chest X-ray upon entering the trial, 5,991 (8.9 per cent) had abnormal results that required follow-up. After undergoing additional tests, 126 (2.1 per cent 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. 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, one must not panic," said Berg.

Berg also noted that tissue variations and other benign factors can resemble tumours on an X-ray. Of the cancers detected, though, 44 per cent 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. 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," said Berg.

According to the release, chest X-rays have not been shown to reduce the death rate from lung cancer, according to several previous studies. Because of their smaller enrolments, 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.

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 per cent) than women (8.2 per cent).