When doctors scan lungs with a special X-ray to find cancer early, they sometimes find cancers that would never have caused harm. But if they watch people for a longer time, fewer of these harmless cancers show up—suggesting that the early scans just found cancers sooner, not that they found more dangerous ones.

When doctors use a special low-dose CT scan to find lung cancer early, about 1 in 5 of the cancers they find might never cause harm — meaning the person would never have known about them if not for the scan, and they wouldn’t need treatment.

When lung cancer is found through a special low-dose CT scan, almost 8 out of 10 of a certain type of tumor might be harmless and would never hurt the person if left alone—only the scan found it, and it wouldn’t have caused any problems otherwise.

When doctors use a low-dose CT scan to check high-risk people for lung cancer, about 1 in 5 of the cancers they find might never have caused any problems — meaning those people could be treated for something that didn’t need treating, leading to stress, side effects, and extra costs.

If you screen 320 people who are at high risk for lung cancer with a special scan, you might save one life—but you’ll also find and treat about 1.4 cancers that would never have hurt them. So for every 100 lives saved, around 138 people get treated for cancers that didn’t need treatment.