One In Three Detected Breast Cancers Is Overdiagnosed

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Main Category: Breast Cancer
Also Included In: Cancer / Oncology
Article Date: 10 Jul 2009 - 0:00 PDT

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Researchers conclude in a paper just published on bmj.com that one in three breast cancers detected in a population with a public breast screening program is overdiagnosed.

Some cancers are harmless and will not cause symptoms or death during a patient's lifetime. The cancer grows so slowly that the patient dies of other causes before it produces symptoms, or the cancer remains dormant or regresses. Overdiagnosis refers to the detection of those cancers.

Since it is impossible to tell apart lethal from harmless cancers, all detected are treated. As a result, overdiagnosis and overtreatment are unavoidable.

Karsten Jørgensen and Peter Gøtzsche at the Nordic Cochrane Centre analyzed breast cancer trends before and after the introduction of publicly organized screening programs, in order to calculate approximately the degree of overdiagnosis. They studied five countries: UK, Canada, Australia, Sweden and Norway.

For the sake of objectivity, they looked at information from seven years before and after screening had been completely implemented in each country. They included both screened and non-screened age groups.

Other factors that may have affected the results were assessed, such as changes in background levels of breast cancer and any compensatory drop in rates of breast cancer among older, previously screened women.

The study showed a rise in occurrence of breast cancer that was directly associated to the introduction of screening. A small proportion of this increase was compensated for by a decrease in incidence of breast cancer in previously screened women.

Subsequently, they evaluated each country and the estimated level of overdiagnosis:

• In the UK: 57 percent.
• In Manitoba: 59 percent.
• In New South Wales: 53 percent.
• In Sweden: 46 percent.
• In Norway: 52 percent.

The total overdiagnosis was estimated at 52 percent. This calculation included a pre-malignant condition known as "carcinoma in situ" that is usually treated with the same method as invasive breast cancer. The over-diagnosis was 35 percent for invasive breast cancer only.

The authors explain that this suggests that one in three breast cancers detected by public breast screening programs are overdiagnosed.

The numbers were computed in the following way: the difference between the observed versus the expected incidence of breast cancer represents overdiagnosis. If the expected incidence based on pre-screening trends is 200 per 100,000 women, but the observed incidence is 300 per 100,000 women then the increase over expected is 100 per 200 (or 50 percent). As a result 100 per 100,000 of 300 per 100,000 observed incidence represents overdiagnosis. This leads to the conclusion that one in three breast cancers are overdiagnosed.

Professor H. Gilbert Welch in an associated editorial writes: "These results are consistent with a growing body of evidence that screening mammography is associated with sustained increases in the incidence of breast cancer in women of screening age, with little or no subsequent decrease in incidence in older women."

He adds that "mammography undoubtedly helps some women but hurts others. No right answer exists; instead it is a personal choice."

He suggests that in order to make a well-informed choice, women need a simple tabular display of benefits and harms based on the latest evidence. There is a need for careful analyses to improve the accuracy of these estimates.

In closing, he recommends that researchers need to take further action rather than just describing the problem. They need to concentrate their efforts by testing higher thresholds, such as only recommending biopsy for breast masses larger than a certain size, in a randomized trial.

"Overdiagnosis in publicly organised mammography screening programmes: systematic review of incidence trends"
Karsten Juhl Jørgensen, researcher Peter C Gøtzsche, director
BMJ 2009; 339:b2587

"Overdiagnosis and mammography screening"
H Gilbert Welch
doi:10.1136/bmj.b2587
bmj.com

Written by Stephanie Brunner (B.A.)
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