CHICAGO — Harmless lung cancer? An analysis suggests the world’s deadliest cancer isn’t as deadly as doctors once thought, finding almost one in five lung tumors detected on CT scans are probably so slow-growing that they would never cause problems.

These were not false positives, or suspicious results that turn out upon further testing not to be cancer. These were indeed cancerous tumors, but ones that caused no symptoms and were unlikely ever to become deadly, the researchers said.

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 tumors show up on an ordinary chest X-ray and are potentially life-threatening.

Also, doctors don’t know yet how to determine which symptomless tumors 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 tumors are harmless and which ones require treatment, said Dr. Edward Patz Jr., lead author and a radiologist at Duke University Medical Center.

Patz said patients who seek lung-cancer screening should be told about the study results.