A significant proportion of cancers detected through screening programs are indolent and will not progress to cause symptoms or death during a person's natural lifespan.

Study: Prevalence of Subclinical Papillary Thyroid Cancer by Age: Meta-analysis of Autopsy Studies.

This study found that many people have tiny thyroid cancers that never caused them any problems — even when they lived to old age. That means if these cancers were found by screening, they probably didn’t need to be treated.

Study: Association between screening and the thyroid cancer “epidemic” in South Korea: evidence from a nationwide study

In South Korea, more people got thyroid cancer diagnoses after doctors started scanning for tiny lumps—but death rates didn’t go up. This means most of those lumps were harmless and would never have hurt anyone, proving that screening often finds cancers that don’t need treatment.

Study: Towards personalized prostate cancer screening

This study says that when men get screened for prostate cancer with a PSA test, many of the cancers found are so slow-growing they’d never hurt the person—even if left untreated. That supports the idea that not all cancers found by screening are dangerous.

Study: Identification of the Fraction of Indolent Tumors and Associated Overdiagnosis in Breast Cancer Screening Trials

This study looked at whether some breast cancers found by screening would never hurt you even if left alone — and found that yes, some probably are harmless, but it’s hard to say exactly how many.