Major analysis suggests breast cancer screening overdiagnosis rates far lower than feared

For decades, women considering breast cancer screening have been told something sobering: the mammogram might find a cancer that would never hurt them, never cause symptoms, never shorten their lives. This possibility—called overdiagnosis—has haunted the screening debate. Some researchers estimated it happened in as many as three or four of every ten cases detected through screening. Those numbers shaped how doctors talked to patients, how health systems designed their programs, and which women chose to participate at all.

Now a major reanalysis of eight landmark randomized trials suggests those estimates were substantially too high. Researchers led by Sisse Helle Njor at the University of Southern Denmark and colleagues at Queen Mary University of London found that when the trial data are examined more carefully—accounting for factors previous analyses had overlooked—the actual rate of overdiagnosis appears to be below 5%. The work, published in JNCI Journal of the National Cancer Institute, challenges interpretations that have circulated through medical literature and public health guidance for years.

The problem was methodological. When screening programs begin, the number of breast cancer diagnoses jumps initially because cancers are caught earlier than they would have been otherwise. Over time, this surge should be followed by a decline, as some of those early-detected cancers would eventually have shown up in unscreened women anyway. But previous analyses often failed to account for what happened after the trials ended. Women in control groups frequently began screening once routine programs were introduced in their regions. The length of follow-up varied between studies. The number of screening rounds offered differed. These variations created a distorted picture. Researchers mistook the temporary spike in diagnoses for evidence of widespread overdiagnosis.

The Danish researchers used an elegant reference point: organized breast cancer screening was introduced in some Danish regions 17 years before others. This natural experiment allowed them to observe exactly how cancer diagnoses changed when screening arrived and how patterns evolved over decades. They compared what they saw in the Danish data with patterns in the eight major trials—the New York Health Insurance Plan, Malmö, Two-County, Edinburgh, the Canadian National Breast Screening Study, Stockholm, Gothenburg, and UK Age. When they aligned the timing and accounted for post-trial screening, follow-up duration, and the number of screening rounds each group received, the trial data matched what Denmark's experience suggested: overdiagnosis below 5%.

Elsebeth Lynge, professor emerita at the University of Copenhagen's Department of Public Health, explained the mechanism. "When screening is introduced, the number of breast cancer diagnoses initially rises because cancers are detected earlier than they would have been without screening. Over time, this should be followed by a drop, as some of these cancers would otherwise have been diagnosed later." The earlier analyses had essentially frozen the picture at the peak, before the decline had time to materialize.

Matejka Rebolj, senior epidemiologist at Queen Mary University of London, was direct about the implications: "Some previous high estimates of overdiagnosis, which influenced screening guidelines and communication, were based on evidence before trial data had fully matured. When interpreted in their full temporal context, randomized trial data are consistent with overdiagnosis of less than 5%, rather than with estimates nearing 50%." That distinction matters enormously. It means the harms of screening—unnecessary biopsies, anxiety, treatment for cancers that would never have caused problems—are far rarer than women have been told.

Njor emphasized what this recalibration means for the women who must decide whether to participate. "Most women will not develop breast cancer, but with this study we can now be reassured that the benefits of detecting breast cancer early and preventing premature death will outweigh the small risk of unnecessary treatment." The finding does not eliminate the risk of overdiagnosis; it relocates it from a substantial threat to a genuine but uncommon occurrence. For screening programs and the doctors who counsel women about them, the challenge now is communicating this more accurate picture—one that acknowledges real harms without inflating them, and that helps women make decisions grounded in what the evidence actually shows.