Breast Cancer Screening: A Provocative Balance Sheet

Imagine if every 50-year-old woman thinking about whether to have a mammogram received the following “balance sheet” showing the “credits” and “debits” as understood by the latest research:

For every 1,000 women who have a yearly mammogram over the next ten years:

Credits

1 woman will avoid dying from breast cancer
Debits
2-10 women will be overdiagnosed and treated needlessly
10-15 women will be told they have breast cancer earlier than they would otherwise have been told, but this will not affect their prognosis
100-500 women will have at least one “false alarm” (about half of these women will undergo a biopsy)

These numbers appear in an editorial in the British Medical Journal in July 2009, written by Dartmouth researcher H. Gilbert Welch, M.D., in response to news that the British National Health Service had scrapped a proposed pamphlet that would have described the benefits of mammograms, but failed to talk about their downside.

The key comparison in the above balance sheet is the ratio of one woman’s life saved for every two to ten lives harmed by overdiagnosis. The one-to-two ratio comes from a study three years ago by Zackrisson and colleagues. The one-to-ten ratio is from a more recent study by Gotzsche. The tipping point for women in deciding whether to undergo annual mammogram screening is probably somewhere on this sliding scale. The test might not feel worthwhile if ten women are hurt for every one helped, but might seem worth it if only two were hurt for every one helped.

As the author writes:

Mammography is one of medicine’s “close calls”-a delicate balance between benefits and harms-where different people in the same situation might reasonably make different choices. Mammography undoubtedly helps some women but hurts others. No right answer exists, instead it is a personal choice.
To inform that choice, women need a simple tabular display of benefit and harms-a balance sheet of credits and debits.

Unfortunately, medicine hasn’t yet pinned down the precise numbers on the balance sheet. For now, women need to be informed that there are varying estimates, and they need to make their own decisions.

Readers should note that this discussion applies only to cancer screening in low-risk women who have no noticeable lumps; women who can feel a lump in their breast are in a different risk category and need to see a doctor promptly.

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