Many lung tumours detected by CT scans are harmless, researchers say

A provocative has study found that nearly one in five lung tumours detected on CT scans are probably so slow-growing they would never cause problems.

These were not false positives, suspicious results that turn out upon further testing not to be lung cancer, the world's No 1 cause of cancer deaths. These were indeed cancerous tumours, but ones that caused no symptoms and were unlikely ever to become deadly.

Still, the results are not likely to change how doctors treat lung cancer.

For one thing, the disease is usually diagnosed after symptoms develop, when tumours show up on an ordinary chest X-ray and are potentially life-threatening. Also, doctors don't know how to tell which symptomless tumours found on CT scans might become dangerous, so they automatically treat the cancer aggressively.

The findings underscore the need to identify biological markers that would help doctors determine which tumours are harmless and which ones require treatment, said Dr Edward Patz, lead author and a radiologist at Duke University Medical Centre. He is among researchers working to do just that.

A leader of an influential government-appointed health panel agreed. "Putting the word 'harmless' next to cancer is such a foreign concept to people," said Dr Michael LeFevre, co-chairman of the US Preventive Services Task Force.

The panel recently issued a draft proposal recommending annual CT scans for high-risk current and former heavy smokers, echoing advice from the American Cancer Society. A final recommendation is pending, but LeFevre said the panel had already assumed that screening might lead to overdiagnosis.

"The more we bring public awareness of this, then the more informed decisions might be when people decide to screen or not," LeFevre said. He called the study "a very important contribution", but said doctors would face a challenge in trying to explain the results to patients.

In testimonials, patients often say lung cancer screening via CT scans led to them being cured, but the study suggests that in many cases, "we cured them of a disease we didn't need to find in the first place", LeFevre said.

The study is published in the JAMA Internal Medicine journal.

Worldwide, there are about 1.5 million lung cancer deaths a year.

An earlier report on the study found that 320 patients would need to get CT screening to prevent one lung cancer death.