A new UK study questions the value of mammograms for breast cancer screening.
It concludes that a woman is more likely to be diagnosed with a small tumour that is not destined to grow than she is to have a true problem spotted early.
The work could further shift the balance of whether screening's harms outweigh its benefits.
Screening is only worthwhile if it finds cancers that would kill, and if treating them early improves survival versus treating when or if they ever cause symptoms.
Treatment has improved so much over the years that detecting cancer early has become less important.
Mammograms do catch some deadly cancers and save lives. But they also find many early cancers that are not destined to grow or spread and become a health threat.
There is no good way to tell which ones will, so many women get treatments they don't really need. It's a twin problem: overdiagnosis and overtreatment.
Whether to have a mammogram ``is a close call, a value judgment,'' said study leader Dr H Gilbert Welch of Dartmouth Medical School.
``This is a choice and it's really important that women understand both sides of the story, the benefits and harms.''
Welch has long argued that mammograms are overrated, and the study parallels work he published from the same data sources four years ago.
This time, the authors include Dr. Barnett Kramer, a National Cancer Institute screening expert, although the conclusions are not an official position of the agency. The study was published Wednesday by the New England Journal of Medicine.
