Much Lower Risk of False Positive Breast Screen in Norway vs US

Frederik Joelving

January 11, 2022

Nearly 1 in 5 women who receive the recommended 10 biennial screening rounds for breast cancer in Norway will get a false positive result, and 1 in 20 women will receive a false positive result that leads to an invasive procedure, a new analysis shows.

While the risk may seem high, it is actually much lower than what researchers have reported in the US, the study authors note in their paper, published online December 21 in Cancer.

"I am proud about the low rate of recalls we have in Norway and Europe — and hope we can keep it that low for the future," said senior author Solveig Hofvind, PhD, head of BreastScreen Norway, a nationwide screening program that invites women aged 50 to 69 to mammographic screening every other year.

"The double reading in Europe is probably the main reason for the lower rate in Europe compared to the US, where single reading is used," she told Medscape Medical News.

Until now, Hofvind and her colleagues say, no studies have been performed using exclusively empirical data to describe the cumulative risk of experiencing a false positive screening result in Europe because of the need for long-term follow-up and complete data registration.

For their study, the researchers turned to the Cancer Registry of Norway, which administers BreastScreen Norway. They focused on data from 1995 to 2019 on women aged 50 to 69 years who had attended one or more screening rounds and could potentially attend all 10 screening examinations over the 20-year period.

Women were excluded if they were diagnosed with breast cancer before attending screening, participated in interventional research, self-referred for screening, were recalled due to self-reported symptoms or technically inadequate mammograms, or declined follow-up after a positive screen.

Among more than 421,000 women who underwent nearly 1.9 million screening examinations, 11.3% experienced at least one false positive result and 3.3% experienced at least one false positive involving an invasive procedure, such as fine-needle aspiration cytology, core-needle biopsy, or open biopsy.

The cumulative risk of experiencing a first false positive screen was 18.0%, and that of experiencing a false positive that involved an invasive procedure was 5.01%. Adjusting for irregular attendance, age at screening, or the number of screens attended had little effect on the estimates.

The results closely match earlier findings from Norway that have been based on assumptions rather than exclusively empirical data. However, these findings differ from results reported in US studies, which have relied largely on data from the Breast Cancer Surveillance Consortium, the researchers say.

"The latter have indicated that, for women who initiate biennial screening at the age of 50 years, the cumulative risk after 10 years is 42% for experiencing at least 1 false-positive screening result and 6.4% for experiencing at least 1 false-positive screening result involving an invasive procedure," Hofvind and her colleagues write.

Several principal investigators with the Breast Cancer Surveillance Consortium did not respond or were unavailable for comment when contacted by Medscape Medical News.

However, the study authors highlighted several factors that could help explain the discrepancy between the US and Europe results.

In addition to double mammogram reading, "European guidelines recommend that breast radiologists read 3500 to 11,000 mammograms annually, whereas 960 every 2 years are required by the US Mammography Quality Standards Act," the researchers note. They also point out that previous screening mammograms are readily available in Norway, whereas this is not always the case in the US.

"False-positive screening results are a part of the screening for breast cancer — and the women need to be informed about the risk," Hofvind concluded. "The screening programs should aim to keep the rate as low as possible for the women [given] the costs."

The study was supported by the Dam Foundation via the Norwegian Breast Cancer Society.

Cancer. Published Online December 21, 2021. Full text

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