Senior woman having a mammography scan at hospital
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Mammograms may only cause about half as many false positives as were earlier estimated, according to a new report from researchers at Duke University and the Fred Hutchinson Cancer Research Center. These scientists used a particular methodology, “combining high-quality current data and refined methods,” to show the shortcomings of earlier studies and set the record straight, they told Inside Precision Oncology.

The lead author is Marc D. Ryser, of Duke University Medical Center. Senior author is Ruth B. Etzioni of the Fred Hutchinson Cancer Research Center. The findings are published in Annals of Internal Medicine.

The researchers performed a modeling study based on data from the Breast Cancer Screening Consortium, the most authoritative data set on breast cancer screening in the US, and found that previous estimates of breast cancer overdiagnosis may have been overestimated. The new model suggests that overdiagnosis, or the finding of tumors that may never progress or cause harm in a woman’s lifetime, occurs in about 15% of screen-detected cancers. The most widely cited estimates previously reached about 30%.

“Part of this is different definitions, study settings, and what technology is used,” said Ryser.

The U.S. Preventive Services Task Force (USPSTF) cites overdiagnosis as one of the chief potential harms associated with mammography screening, because of the burden and adverse consequences of unnecessary treatments.

Approximately 7 in 1,000 women are diagnosed with invasive or noninvasive breast cancer on the basis of a screening mammogram annually. But according to this estimate, 1 in 1,000 women have cancers that would never have caused problems. Assuming that about 60% of the 280,000 cases of breast cancer diagnosed in the United States each year are found through mammography screening, eliminating overdiagnosis could spare 25,000 women the cost and complications of unnecessary treatment. Therefore, knowledge about overdiagnosis is critical for supporting shared decision making about screening.

The researchers studied Breast Cancer Surveillance Consortium facility data to estimate the rate of breast cancer overdiagnosis in contemporary mammography practice for a cohort including 35,986 women, 82,677 mammograms, and 718 breast cancer diagnoses. To estimate overdiagnosis, the authors considered a cohort of women whose disease natural history was given by the best-fitting parameter combinations and who had annual or biennial screening, starting at age 50 years and until age 74 years or death from a cause unrelated to breast cancer, whichever occurred first.

They modeled the competing mortality risk on the basis of a published age–cohort model for a 1971 birth cohort. The researchers found that in a program of biennial screening of women aged 50 to 74 years, which corresponds to USPSTF recommendations for average risk women, approximately 1 in 7 screen-detected cases would be overdiagnosed. Increasing the screening interval to annual screening did not seem to affect this number.

An accompanying editorial from Massachusetts General Hospital says that these findings may help women who are considering having mammography screening better understand the risk of overdiagnosis.

Another key question is why don’t some breast cancers progress? And can they be identified?  Would “watchful waiting,” be more appropriate than treatment, as is done for some prostate cancers?

Ductal carcinoma in situ (DCIS) is a non-invasive precursor of more serious breast cancer.  Ryser and Etzioni point to a study being led by Duke’s E. Shelley Wang, another co-author on the current paper.  That research, The Comparison of Operative versus Monitoring and Endocrine Therapy (COMET) trial is evaluating treatment of these patients and trying to determine if some would do better with active surveillance than operative intervention.

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