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How mass breast cancer screening failed to diagnose serious cases, in one chart

Incidence of cancer that was metastatic at first presentation, 1975-2012.
Incidence of cancer that was metastatic at first presentation, 1975 to 2012. (NEJM)

The whole idea behind cancer screening is that doctors will find the disease earlier — even before symptoms appear — and therefore be more likely to successfully treat it and control any spread.

But as this chart, from the latest issue of the New England Journal of Medicine, shows, the introduction of mass mammography screening has failed to reduce the rate of advanced-stage breast cancers in women.

As you can see, the incidence of metastatic cancers (or cancers that have already spread and are therefore more deadly) has remained stable since 1975 — despite the initiation of widespread screening programs aimed at getting all women of a certain age mammograms. (I'll get to the prostate cancer trend in a moment.)

The stubborn trend suggests, lead author and Dartmouth physician Gilbert Welch said, that mammography programs aren't having any impact on the number of women who have serious breast cancers, which means mass screening probably can't detect the most aggressive forms of the disease at an early stage.

Interestingly, the authors of the paper note, the average age at diagnosis of metastatic breast cancer among women 40 and older hasn’t changed (remaining 64 years) in nearly 40 years.

This isn't the first time researchers have questioned the utility of population-wide mammography. Some medical associations and cancer societies have pushed back the initiation of mass screening, from about 40 years to 45 or 50 years. These general guidelines don't apply to women who have a family history of the disease, or lifestyle factors and genetic markers that put them at a higher risk. And it should be noted that mass screening is different from using technologies like ultrasounds to diagnose individuals at risk of a disease or who have symptoms that require investigation.

Prostate cancer screening is more complex

As the chart shows, the story is slightly different for prostate cancer: It appears mass screening has been able to detect the more serious metastatic cases early and thus reduce the number of people with cancers that eventually spread.

The researchers think this is because the screening methodology — a prostate-specific antigen test accompanied by multiple biopsies — is more sensitive than the mammogram. Here, the average age at diagnosis for men over 40 fell by two years, from 71.8 to 69.8, following mass screening.

This sensitivity has its downsides: Many men who would never die from the disease get diagnosed. Welch also noted that it's unclear whether this extra detection has an impact on things that matter to patients, like mortality or disability. But it does appear that the screening programs are catching cancers that could spread early.

In the paper, the authors note that the situation for breast cancer screening may improve in the future with the development of more sensitive screening tools for women:

Although these discordant trends could reflect distinct disease dynamics, they could also be the result of different screening strategies. Mammography represents an anatomical search for a structural abnormality; PSA screening uses a biochemical assay to detect a tumor marker. It's possible that the latter is a much more sensitive indicator of disease burden.

Were a similar breast-cancer assay discovered — and a similar organwide sampling strategy used (the typical prostate biopsy now involves at least 10 needle cores throughout the organ) — then perhaps fewer women would present with metastatic breast cancer.

"Again," they conclude, "whether mortality would therefore decline is a separate question."